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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.

Summary Bipolar disorder (BD) is characterized by cyclical mood shifts. Studies indicate that BD patients have a peripheral pro-inflammatory state and alterations in glial populations in the brain. We utilized an in vitro model to study inflammation-related phenotypes of astrocytes derived from induced pluripotent stem cells (iPSCs) generated from BD patients and healthy controls. BD astrocytes showed changes in transcriptome and induced a reduction in neuronal activity when co-cultured with neurons. IL-1β-stimulated BD astrocytes displayed a unique inflammatory gene expression signature and increased secretion of IL-6. Conditioned medium from stimulated BD astrocytes reduced neuronal activity, and this effect was partially blocked by IL-6 inactivating antibody. Our results suggest that BD astrocytes are functionally less supportive of neuronal excitability and this effect is partially mediated by IL-6. We confirmed higher IL-6 in blood in a distinct cohort of BD patients, highlighting the potential role of astrocyte-mediated inflammatory signaling in BD neuropathology., Highlights • Bipolar disorder astrocytes are functionally less supportive of neuronal activity • Bipolar disorder astrocytes response to IL-1β is transcriptionally distinct • IL-6 secretion in bipolar disorder astrocytes reduces neuronal activity • Bipolar disorder patients show higher circulating levels of IL-6 in blood, In this article, Gage and collaborators show that astrocytes differentiated from induced pluripotent stem cells generated from bipolar disorder patients are functionally less supportive of neuronal activity. Bipolar disorder astrocytes' response to pro-inflammatory cytokines is characterized by a unique transcriptional response and increased IL-6 secretion that directly and negatively impacted on neuronal activity. Increased peripheral IL-6 was confirmed in a distinct clinical cohort highlighting the potential role of astrocyte-mediated inflammatory signaling in the neuropathology of bipolar disorder.

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.

Chemical modifications of histones can mediate diverse DNA-templated processes, including gene transcription1–3. Here we provide evidence for a class of histone post-translational modification, serotonylation of glutamine, which occurs at position 5 (Q5ser) on histone H3 in organisms that produce serotonin (also known as 5-hydroxytryptamine (5-HT)). We demonstrate that tissue transglutaminase 2 can serotonylate histone H3 tri-methylated lysine 4 (H3K4me3)-marked nucleosomes, resulting in the presence of combinatorial H3K4me3Q5ser in vivo. H3K4me3Q5ser displays a ubiquitous pattern of tissue expression in mammals, with enrichment observed in brain and gut, two organ systems responsible for the bulk of 5-HT production. Genome-wide analyses of human serotonergic neurons, developing mouse brain and cultured serotonergic cells indicate that H3K4me3Q5ser nucleosomes are enriched in euchromatin, are sensitive to cellular differentiation and correlate with permissive gene expression, phenomena that are linked to the potentiation of TFIID4–6 interactions with H3K4me3. Cells that ectopically express a H3 mutant that cannot be serotonylated display significantly altered expression of H3K4me3Q5ser-target loci, which leads to deficits in differentiation. Taken together, these data identify a direct role for 5-HT, independent from its contributions to neurotransmission and cellular signalling, in the mediation of permissive gene expression. In serotonin-rich tissues, tissue transglutaminase 2 is able to attach serotonin to a glutamine residue in histone H3; this modification mediates permissive gene expression in these tissues.

Serotonin (5-HT) is an evolutionarily ancient neurotransmitter that has long been associated with psychiatric disease, including major depression disorder (MDD). Though it is a genetically and clinically heterogeneous disorder, MDD is most often treated with selective serotonin reuptake inhibitors (SSRIs) that target the serotonergic neurotransmitter system. While this antidepressant intervention is beneficial for some, nearly 30% of MDD patients do not respond to SSRIs, and it remains unclear how alterations in serotonergic neurotransmission impact SSRI-treatment resistance in MDD. In vitro technological advances now allow for adult patient somatic cells to be reprogrammed to induced pluripotent stem cells, which can then be differentiated to generate neural cell subtypes relevant to the study of psychiatric disorders. We, along with other labs, have pioneered methods for generating and assaying human serotonergic neurons. Here, we describe these methods and summarize our research findings on altered serotonergic neurotransmission in SSRI-resistant MDD patients.

WOS: 000458101700001 PubMed ID: 30730291 Comparative analyses of neuronal phenotypes in closely related species can shed light on neuronal changes occurring during evolution. The study of post-mortem brains of nonhuman primates (NHPs) has been limited and often does not recapitulate important species-specific developmental hallmarks. We utilize induced pluripotent stem cell (iPSC) technology to investigate the development of cortical pyramidal neurons following migration and maturation of cells grafted in the developing mouse cortex. Our results show differential migration patterns in human neural progenitor cells compared to those of chimpanzees and bonobos both in vitro and in vivo, suggesting heterochronic changes in human neurons. The strategy proposed here lays the groundwork for further comparative analyses between humans and NHPs and opens new avenues for understanding the differences in the neural underpinnings of cognition and neurological disease susceptibility between species. National Institutes of Health; California Institute for Regenerative Medicine; National Alliance for Research on Schizophrenia and Depression; Leona M. and Harry B. Helmsley Charitable Trust; Salk Cancer Center [NCI P30 CA014195] National Institutes of Health Research Project Grant Maria C Marchetto Alysson R Muotri Krishnan Padmanabhan Fred H Gage; California Institute for Regenerative Medicine Research Grant Alysson R Muotri; National Institutes of Health Pathway to independence award Krishnan Padmanabhan; National Alliance for Research on Schizophrenia and Depression Young Investigator award Krishnan Padmanabhan; Leona M. and Harry B. Helmsley Charitable Trust Research Project Grant Fred H Gage Salk Cancer Center NCI P30 CA014195 Fred H Gage; The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.

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.

Spondylocarpotarsal synostosis (SCT) is a skeletal disorder characterized by progressive vertebral, carpal and tarsal fusions, and mild short stature. The majority of affected individuals have an autosomal recessive form of SCT and are homozygous or compound heterozygous for nonsense mutations in the gene that encodes the cytoskeletal protein filamin B (FLNB), but a subset do not have FLNB mutations. Exome sequence analysis of three SCT patients negative for FLNB mutations identified an autosomal dominant form of the disease due to heterozygosity for missense or nonsense mutations in MYH3, which encodes embryonic myosin. Cells transfected with the MYH3 missense mutations had reduced TGFβ signaling, revealing a regulatory role for embryonic myosin in the TGFβ signaling pathway. In wild-type mice, there was persistent postnatal expression of embryonic myosin in the small muscles joining the neural arches of the spine suggesting that loss of myosin function in these muscles contribute to the disease. Our findings demonstrate that dominant mutations in MYH3 underlie autosomal dominant SCT, identify a postnatal role for embryonic myosin and suggest that altered regulation of signal transduction in the muscles within the spine may lead to the development of vertebral fusions.