ER stress in mouse serotonin neurons triggers a depressive phenotype alleviated by ketamine targeting eIF2α signaling.

Depression is a devastating mood disorder that causes significant disability worldwide. Current knowledge of its pathophysiology remains modest and clear biological markers are lacking. Emerging evidence from human and animal models reveals persistent alterations in endoplasmic reticulum (ER) homeostasis, suggesting that ER stress-related signaling pathways may be targets for prevention and treatment. However, the neurobiological basis linking the pathways involved in depression-related ER stress remains unknown. Here, we report that an induced model of ER stress in mouse serotonin (5-HT) neurons is associated with reduced Egr1-dependent 5-HT cellular activity and 5-HT neurotransmission, resulting in neuroplasticity deficits in forebrain regions and a depressive-like phenotype. Ketamine administration engages downstream eIF2α signaling to trigger rapid neuroplasticity events that rescue the depressive-like effects. Collectively, these data identify ER stress in 5-HT neurons as a cellular pathway involved in the pathophysiology of depression and show that eIF2α is critical in eliciting ketamine's fast antidepressant effects.

Oligonucleotides as therapeutic tools for brain disorders: Focus on major depressive disorder and Parkinson's disease.

Remarkable advances in understanding the role of RNA in health and disease have expanded considerably in the last decade. RNA is becoming an increasingly important target for therapeutic intervention; therefore, it is critical to develop strategies for therapeutic modulation of RNA function. Oligonucleotides, including antisense oligonucleotide (ASO), small interfering RNA (siRNA), microRNA mimic (miRNA), and anti-microRNA (antagomir) are perhaps the most direct therapeutic strategies for addressing RNA. Among other mechanisms, most oligonucleotide designs involve the formation of a hybrid with RNA that promotes its degradation by activation of endogenous enzymes such as RNase-H (e.g., ASO) or the RISC complex (e.g. RNA interference - RNAi for siRNA and miRNA). However, the use of oligonucleotides for the treatment of brain disorders is seriously compromised by two main limitations: i) how to deliver oligonucleotides to the brain compartment, avoiding the action of peripheral RNAses? and once there, ii) how to target specific neuronal populations? We review the main molecular pathways in major depressive disorder (MDD) and Parkinson's disease (PD), and discuss the challenges associated with the development of novel oligonucleotide therapeutics. We pay special attention to the use of conjugated ligand-oligonucleotide approach in which the oligonucleotide sequence is covalently bound to monoamine transporter inhibitors (e.g. sertraline, reboxetine, indatraline). This strategy allows their selective accumulation in the monoamine neurons of mice and monkeys after their intranasal or intracerebroventricular administration, evoking preclinical changes predictive of a clinical therapeutic action after knocking-down disease-related genes. In addition, recent advances in oligonucleotide therapeutic clinical trials are also reviewed.

Ucn3 and CRF-R2 in the medial amygdala regulate complex social dynamics.

Social encounters are associated with varying degrees of emotional arousal and stress. The mechanisms underlying adequate socioemotional balance are unknown. The medial amygdala (MeA) is a brain region associated with social behavior in mice. Corticotropin-releasing factor receptor type-2 (CRF-R2) and its specific ligand urocortin-3 (Ucn3), known components of the behavioral stress response system, are highly expressed in the MeA. Here we show that mice deficient in CRF-R2 or Ucn3 exhibit abnormally low preference for novel conspecifics. MeA-specific knockdown of Crfr2 (Crhr2) in adulthood recapitulated this phenotype. In contrast, pharmacological activation of MeA CRF-R2 or optogenetic activation of MeA Ucn3 neurons increased preference for novel mice. Furthermore, chemogenetic inhibition of MeA Ucn3 neurons elicited pro-social behavior in freely behaving groups of mice without affecting their hierarchal structure. These findings collectively suggest that the MeA Ucn3-CRF-R2 system modulates the ability of mice to cope with social challenges.

Dnmt3a in the Medial Prefrontal Cortex Regulates Anxiety-Like Behavior in Adult Mice.

Recently, it has been suggested that alterations in DNA methylation mediate the molecular changes and psychopathologies that can occur following trauma. Despite the abundance of DNA methyltransferases (Dnmts) in the brain, which are responsible for catalyzing DNA methylation, their roles in behavioral regulation and in response to stressful challenges remain poorly understood. Here, we demonstrate that adult mice which underwent chronic social defeat stress (CSDS) displayed elevated anxiety-like behavior that was accompanied by a reduction in medial prefrontal cortex (mPFC)-DNA methyltransferase 3a (Dnmt3a) mRNA levels and a subsequent decrease in mPFC-global DNA methylation. To explore the role of mPFC-Dnmt3a in mediating the behavioral responses to stressful challenges we established lentiviral-based mouse models that express lower (knockdown) or higher (overexpression) levels of Dnmt3a specifically within the mPFC. Nonstressed mice injected with knockdown Dnmt3a lentiviruses specifically into the mPFC displayed the same anxiogenic phenotype as the CSDS mice, whereas overexpression of Dnmt3a induced an opposite, anxiolytic, effect in wild-type mice. In addition, overexpression of Dnmt3a in the mPFC of CSDS mice attenuated stress-induced anxiety. Our results indicate a central role for mPFC-Dnmt3a as a mediator of stress-induced anxiety. Significance statement: DNA methylation is suggested to mediate the molecular mechanisms linking environmental challenges, such as chronic stress or trauma, to increased susceptibility to psychopathologies. Here, we show that chronic stress-induced increase in anxiety-like behavior is accompanied by a reduction in DNA methyltransferase 3a (Dnmt3a) mRNA levels and global DNA methylation in the medial prefrontal cortex (mPFC). Overexpression or knockdown of mPFC-Dnmt3a levels induces decrease or increase in anxiety-like behavior, respectively. In addition, overexpression of Dnmt3a in the mPFC of chronic stressed mice attenuated stress-induced anxiety. We suggest that mPFC-Dnmt3a levels mediates anxiety-like behavior, which may be a primary molecular link between chronic stress and the development of anxiety disorders, including post-traumatic stress disorder.