Dysregulated mammalian target of rapamycin (mTOR) activity is associated with various neurodevelopmental disorders ranging from idiopathic autism spectrum disorders (ASD) to syndromes caused by single gene defects. This suggests that maintaining mTOR activity levels in a physiological range is essential for brain development and functioning. Upon activation, mTOR regulates a variety of cellular processes such as cell growth, autophagy, and metabolism. On a molecular level, however, the consequences of mTOR activation in the brain are not well understood. Low levels of cholesterol are associated with a wide variety of neurodevelopmental disorders. We here describe numerous genes of the sterol/cholesterol biosynthesis pathway to be transcriptionally regulated by mTOR complex 1 (mTORC1) signaling in vitro in primary neurons and in vivo in the developing cerebral cortex of the mouse. We find that these genes are shared targets of the transcription factors SREBP, SP1, and NF-Y. Prenatal as well as postnatal mTORC1 inhibition downregulated expression of these genes which directly translated into reduced cholesterol levels, pointing towards a substantial metabolic function of the mTORC1 signaling cascade. Altogether, our results indicate that mTORC1 is an essential transcriptional regulator of the expression of sterol/cholesterol biosynthesis genes in the developing brain. Altered expression of these genes may be an important factor contributing to the pathogenesis of neurodevelopmental disorders associated with dysregulated mTOR signaling.

Chronic stress has become a predominant factor associated with a variety of psychiatric disorders, such as depression and anxiety, in both human and animal models. Although multiple studies have looked at transcriptional changes after social defeat stress, these studies primarily focus on bulk tissues, which might dilute important molecular signatures of social interaction in activated cells. In this study, we employed the Arc-GFP mouse model in conjunction with chronic social defeat (CSD) to selectively isolate activated nuclei (AN) populations in the ventral hippocampus (vHIP) and prefrontal cortex (PFC) of resilient and susceptible animals. Nuclear RNA-seq of susceptible vs. resilient populations revealed distinct transcriptional profiles linked predominantly with neuronal and synaptic regulation mechanisms. In the vHIP, susceptible AN exhibited increased expression of genes related to the cytoskeleton and synaptic organization. At the same time, resilient AN showed upregulation of cell adhesion genes and differential expression of major glutamatergic subunits. In the PFC, susceptible mice exhibited upregulation of synaptotagmins and immediate early genes (IEGs), suggesting a potentially over-amplified neuronal activity state. Our findings provide a novel view of stress-exposed neuronal activation and the molecular response mechanisms in stress-susceptible vs. resilient animals, which may have important implications for understanding mental resilience.

MicroRNAs are particularly important during brain development, however, the composition and temporal dynamics of miRNA regulatory networks are not sufficiently characterized. Here, we performed small RNA sequencing of mouse embryonic cortical samples at E14, E17, and P0 as well as in neural progenitor cells differentiated in vitro into neurons. Using co-expression network analysis, we detected clusters of miRNAs that were co-regulated at distinct developmental stages. miRNAs such as miR-92a/b acted as hubs during early, and miR-124 and miR-137 during late neurogenesis. Notably, validated targets of P0 hub miRNAs were enriched for down-regulated genes related to stem cell proliferation, negative regulation of neuronal differentiation and RNA splicing, among others, suggesting that miRNAs are particularly important for modulating transcriptional programs of crucial factors that guide the switch to neuronal differentiation. As most genes contain binding sites for more than one miRNA, we furthermore constructed a co-targeting network where numerous miRNAs shared more targets than expected by chance. Using luciferase reporter assays, we demonstrated that simultaneous binding of miRNA pairs to neurodevelopmentally relevant genes exerted an enhanced transcriptional silencing effect compared to single miRNAs. Taken together, our study provides a comprehensive resource of miRNA longitudinal expression changes during corticogenesis. Furthermore, we highlight several potential mechanisms through which miRNA regulatory networks can shape embryonic brain development.

Susceptibility to chronic social stressors often results in the development of mental health disorders including major depressive and anxiety disorders. In contrast, some individuals remain resilient even after repeated stress exposure. Understanding the molecular drivers underlying these divergent phenotypic outcomes is crucial. However, previous studies using the chronic social defeat (CSD) stress model have been limited by the use of bulk tissues investigating single omics domains. To overcome these limitations, here, we applied the CSD mouse model to Arc-GFP mice for investigating the mechanistic divergence between susceptibility and resilience, specifically in stress recall-activated nuclei. By conducting an in-depth analysis of the less-known differential methylome landscape in the ventral hippocampus, we noted unique phenotype-specific alterations in multiple biological processes with an overrepresentation of GTPase-related mechanisms. Interestingly, the differentially methylated regions were enriched in ETS transcription factor binding sites (TFBSs), important targets of the Ras-ETS signaling pathway. This differential methylation in the ETS TFBSs could form the basis of persisting stress effects long after stressor exposure. Furthermore, by integrating the methylome modifications with transcriptomic alterations, we delineated the GTPase-related mechanisms differentially activated in the resilient and susceptible phenotypes with alterations in endocytosis overrepresented in the susceptible phenotype. Overall, our findings reveal novel insights underlying the resilience-susceptibility divergence.

Susceptibility to chronic social stressors often results in the development of mental health disorders including major depressive and anxiety disorders. In contrast, some individuals remain resilient even after repeated stress exposure. Understanding the molecular drivers behind these divergent phenotypic outcomes is crucial. However, previous studies using the chronic social defeat (CSD) stress model have been limited by the use of bulk tissues investigating single omics domains. To overcome these limitations, here, we applied the CSD mouse model to Arc-GFP mice for investigating the mechanistic divergence between susceptibility and resilience, specifically in stress recall-activated engram nuclei. By conducting an in-depth analysis of the less-known differential methylome landscape in the ventral hippocampal engrams, we noted unique phenotype-specific alterations in multiple biological processes with an overrepresentation of GTPase-related mechanisms. Interestingly, the differentially methylated regions were enriched in ETS transcription factor binding sites (TFBSs), important targets of the Ras-ETS signaling pathway. This differential methylation in the ETS TFBSs could form the basis of persisting stress effects long after stressor exposure. Furthermore, by integrating the methylome modifications with transcriptomic alterations, we resolved the GTPase-related mechanisms differentially activated in the resilient and susceptible phenotypes with alterations in endocytosis overrepresented in the susceptible phenotype. Overall, our findings implicate critical avenues for future therapeutic applications.

Dysregulated mammalian target of rapamycin (mTOR) activity is associated with various neurodevelopmental disorders ranging from idiopathic autism spectrum disorders to syndromes caused by single gene defects. This suggests that maintaining mTOR activity levels in a physiological range is essential for brain development and functioning. Upon activation, mTOR regulates a variety of cellular processes such as cell growth, autophagy and metabolism. On a molecular level, however, the consequences of mTOR activation in the brain are not well understood. Low levels of cholesterol are associated with a wide variety of neurodevelopmental disorders. We here describe numerous genes of the sterol/cholesterol biosynthesis pathway to be transcriptionally regulated by mTOR complex 1 (mTORC1) signaling in vitro in primary neurons and in vivo in the developing cerebral cortex of the mouse. We find that these genes are shared targets of the transcription factors SREBP, SP1 and NF-Y. Prenatal as well as postnatal mTORC1 inhibition downregulated expression of these genes which directly translated into reduced cholesterol levels pointing towards a substantial metabolic function of the mTORC1 signaling cascade. Altogether, our results indicate that mTORC1 is an essential transcriptional regulator of the expression of sterol/cholesterol biosynthesis genes in the developing brain. Altered expression of these genes may be an important factor contributing to the pathogenesis of neurodevelopmental disorders associated with dysregulated mTOR signaling.