Post by Deborah Joye
What's the science?
Selective serotonin reuptake inhibitors (SSRIs) are the most common medication prescribed for Major Depressive Disorder (MDD). However, 30% of MDD patients do not improve with this type of medication and the reason why is unclear. One powerful technique to study this problem in patient populations is induced pluripotent stem cell (iPSC) technology (see BrainPost’s explanation of iPSC). iPSC technology has been instrumental for disorders such as schizophrenia and bipolar disorder, but has not yet been used to study MDD or treatment-resistant depression. This week in Molecular Psychiatry, Vadodaria and colleagues use iPSC technology to study serotonergic neurotransmission in neurons from patients who respond to SSRI treatment and those who do not, to demonstrate that SSRI neurons from non-responders become hyperactive in response to serotonin, which may play a role in SSRI resistance.
How did they do it?
The authors selected extreme cases of SSRI responders and non-responders from a large group of 803 MDD patients involved in an 8-week Pharmacogenomic Research Network Antidepressant Medication Pharmacogenomic Study. Depressive symptoms were quantified using the quick inventory of depressive symptomatology (QIDS) and Hamilton depression (HAMD) rating scales before beginning the 8-week study. Patients who exhibited remission from depressive symptoms after 8 weeks of SSRI treatment were considered responders; patients who did not exhibit changes in QIDS and HAMD scores were considered non-responders. The authors collected skin biopsies from 3 responsive, 3 non-responsive, and 3 healthy control subjects and used iPSC technology to reprogram skin cells into neurons. They then reprogrammed patient-derived iPSCs to become neurons from the forebrain, a region known to respond to serotonin signals.
To investigate activity levels of patient-derived neurons, the authors used a calcium-responsive dye to image calcium inside the cells. Electrical activation of neurons typically results in increases in intracellular calcium, allowing the authors to observe and measure calcium dynamics at baseline, and subsequently after treatment with serotonin (mimicking the actions of SSRI treatment). Since serotonin mediates its effects via seven known families of receptors, the authors then used receptor antagonists (to block specific receptors) and western blot (detects proteins present in tissue) to determine which receptors were responsible for mediating the effects of serotonin in SSRI responder, non-responder, and healthy neurons. Finally, the authors used RNA sequencing to analyze differences in protein expression between responder, non-responder, and healthy neurons.
What did they find?
First, the authors visualized calcium dynamics and found no differences between groups at baseline; however, after exposure to serotonin, non-responder neurons exhibited significantly higher activity compared to responder and healthy control neurons. This suggests that serotonin-induced hyperactivity may play a role in SSRI treatment resistance. Using RNA sequencing, the authors found that transcripts for two serotonin receptors – 5-HT2A and 5-HT7 – were differentially expressed between responder and non-responder groups. To confirm this, the authors analyzed proteins in patient-derived neurons using western blot and found significantly higher levels of 5-HT2A and 5-HT7 receptors in non-responder neurons compared to both healthy and responder neurons. This suggests that some receptors may be upregulated in non-responders. Since both receptors result in excitatory actions on neurons, the authors used specific antagonists to block signaling at 5-HT2A and 5-HT7 receptors and found that this reduced the hyperactive response to serotonin.
What's the impact?
This study is the first to utilize iPSC technology to study patient-derived neurons from individuals with MDD. The findings of this study suggest that forebrain neurons from SSRI-treatment-resistant patients become hyperactive in response to serotonin – an effect that is mediated by upregulation of two specific serotonin receptors, 5-HT2A and 5-HT7. This study decisively moves MDD neuroscience research forward by utilizing cutting-edge iPSC technology to investigate patient-derived neuronal response to therapeutics already on the market.
A word of caution: It should be noted that the sample in this study is comprised entirely of females and examining sex differences in SSRI treatment response as well as general MDD neural circuits presents exciting avenues for future research.
Vadodaria et al., Serotonin-induced hyperactivity in SSRI-resistant major depressive disorder patient-derived neurons, Molecular Psychiatry (2019), Access the original scientific publication here.