Chronic cortisol differentially impacts stem cell-derived astrocytes from major depressive disorder patients.

Major depressive disorder (MDD) is a prevalent psychiatric disorder, and exposure to stress is a robust risk factor for MDD. Clinical data and rodent models have indicated the negative impact of chronic exposure to stress-induced hormones like cortisol on brain volume, memory, and cell metabolism. However, the cellular and transcriptomic changes that occur in the brain after prolonged exposure to cortisol are less understood. Furthermore, the astrocyte-specific contribution to cortisol-induced neuropathology remains understudied. Here, we have developed an in vitro model of "chronic stress" using human induced pluripotent stem cell (iPSC)-derived astrocytes treated with cortisol for 7 days. Whole transcriptome sequencing reveals differentially expressed genes (DEGs) uniquely regulated in chronic cortisol compared to acute cortisol treatment. Utilizing this paradigm, we examined the stress response transcriptome of astrocytes generated from MDD patient iPSCs. The MDD-specific DEGs are related to GPCR ligand binding, synaptic signaling, and ion homeostasis. Together, these data highlight the unique role astrocytes play in the central nervous system and present interesting genes for future study into the relationship between chronic stress and MDD.

Altered serotonergic circuitry in SSRI-resistant major depressive disorder patient-derived neurons.

Disrupted serotonergic neurotransmission has long been implicated in major depressive disorder (MDD), for which selective serotonin reuptake inhibitors (SSRIs) are the first line of treatment. However, a significant percentage of patients remain SSRI-resistant and it is unclear whether and how alterations in serotonergic neurons contribute to SSRI resistance in these patients. Induced pluripotent stem cells (iPSCs) facilitate the study of patient-specific neural subtypes that are typically inaccessible in living patients, enabling the discovery of disease-related phenotypes. In our study of a well-characterized cohort of over 800 MDD patients, we generated iPSCs and serotonergic neurons from three extreme SSRI-remitters (R) and SSRI-nonremitters (NR). We studied serotonin (5-HT) biochemistry and observed no significant differences in 5-HT release and reuptake or in genes related to 5-HT biochemistry. NR patient-derived serotonergic neurons exhibited altered neurite growth and morphology downstream of lowered expression of key Protocadherin alpha genes as compared to healthy controls and Rs. Furthermore, knockdown of Protocadherin alpha genes directly regulated iPSC-derived neurite length and morphology. Our results suggest that intrinsic differences in serotonergic neuron morphology and the resulting circuitry may contribute to SSRI resistance in MDD patients.

Serotonin-induced hyperactivity in SSRI-resistant major depressive disorder patient-derived neurons.

Selective serotonin reuptake inhibitors (SSRIs) are the most prescribed antidepressants. They regulate serotonergic neurotransmission, but it remains unclear how altered serotonergic neurotransmission may contribute to the SSRI resistance observed in approximately 30% of major depressive disorder (MDD) patients. Patient stratification based on pharmacological responsiveness and the use of patient-derived neurons may make possible the discovery of disease-relevant neural phenotypes. In our study from a large cohort of well-characterized MDD patients, we have generated induced pluripotent stem cells (iPSCs) from SSRI-remitters and SSRI-nonremitters. We studied serotonergic neurotransmission in patient forebrain neurons in vitro and observed that nonremitter patient-derived neurons displayed serotonin-induced hyperactivity downstream of upregulated excitatory serotonergic receptors, in contrast to what is seen in healthy and remitter patient-derived neurons. Our data suggest that postsynaptic forebrain hyperactivity downstream of SSRI treatment may play a role in SSRI resistance in MDD.