The GR–RACK1 Axis as a Molecular Link Between Chronic Mild Stress and Neurodegenerative Susceptibility.

Abstract (English) Chronic stress and major depressive disorder are associated with dysregulation of the hypothalamic-pituitary-adrenal axis, prolonged glucocorticoid exposure and impaired neuroplasticity. These alterations have also been implicated in the increased vulnerability to cognitive decline and Alzheimer’s disease. Among the molecular mechanisms potentially linking stress-related disorders to neurodegeneration, the glucocorticoid receptor/RACK1/BDNF axis has emerged as a relevant pathway. RACK1 is a multifunctional scaffold and ribosome-associated protein involved in signal transduction, translational regulation and neuroplasticity-related processes. Previous evidence indicates that RACK1 expression is regulated by glucocorticoids through glucocorticoid receptor-dependent mechanisms and that RACK1 may contribute to BDNF promoter regulation. This thesis investigated whether alterations in glucocorticoid receptor alternative splicing, particularly through the SRSF3/GRα pathway, may contribute to RACK1 dysregulation in stress-related and Alzheimer’s disease-associated conditions. To address this aim, molecular alterations were analysed in a chronic mild stress model, in differentiated SH-SY5Y cells exposed to cortisol or amyloid-β peptides, in the cortex of Tg2576 mice, and in human Alzheimer’s disease cortical samples and transcriptomic datasets. In the chronic mild stress model, vulnerable animals showed increased circulating corticosterone levels, increased SRSF3 expression and reduced RACK1 protein levels, whereas resilient animals displayed increased SRSF9 and GRβ expression together with preserved or increased RACK1 levels. In differentiated SH-SY5Y cells, cortisol increased SRSF3 expression, favoured GRα-dependent signalling and reduced RACK1 expression at both mRNA and protein levels. Silencing of SRSF3 or GRα, as well as pharmacological inhibition of GR, prevented cortisol-induced RACK1 downregulation, supporting a functional role for the SRSF3/GRα axis in this mechanism. In Alzheimer’s disease-related models, amyloid-associated pathology was accompanied by increased SRSF3 and GRα expression and reduced RACK1 protein levels. Importantly, RACK1 protein was also significantly reduced in the cortex of Alzheimer’s disease patients. However, analysis of human cortical transcriptomic datasets did not show a corresponding decrease in RACK1 mRNA levels, indicating that the in vitro model does not fully recapitulate the complexity of human Alzheimer’s disease. This discrepancy suggests that RACK1 dysregulation in Alzheimer’s disease may involve additional post-transcriptional, translational or protein stability mechanisms. Overall, these findings support the hypothesis that SRSF3 and GRα may act as molecular mediators of an unfavourable cellular environment that ultimately affects RACK1 protein expression. Further studies using more complex Alzheimer’s disease models will be required to better dissect this pathway and clarify its relevance in neurodegenerative vulnerability.