The cholinergic interneurons (ChIs) of the nucleus accumbens (NAc) have a critical role in the activity of this region, specifically in the context of major depressive disorder. To understand the circuitry regulating this behavior, we sought to determine the areas that directly project to these interneurons by utilizing the monosynaptic cell-specific tracing technique. Mapping showed monosynaptic projections that are exclusive to NAc ChIs. To determine if some of these projections are altered in a depression mouse model, we used mice that do not express the calcium-binding protein p11 specifically in ChIs (ChAT-p11 cKO) and display a depressive-like phenotype. Our data demonstrated that while the overall projection areas remain similar between wild type and ChAT-p11 cKO mice, the number of projections from the ventral hippocampus (vHIP) is significantly reduced in the ChAT-p11 cKO mice. Furthermore, using optogenetics and electrophysiology we showed that glutamatergic projections from vHIP to NAc ChIs are severely altered in mutant mice. These results show that specific alterations in the circuitry of the accumbal ChIs could play an important role in the regulation of depressive-like behavior, reward-seeking behavior in addictions, or psychiatric symptoms in neurodegenerative diseases.

BACKGROUND:Highly palatable food triggers behavioral alterations reminiscent of those induced by addictive drugs. These effects involve the reward system and dopamine neurons, which modulate neurons in the nucleus accumbens (NAc). The molecular mechanisms underlying the effects of highly palatable food on feeding behavior are poorly understood.METHODS:We studied the effects of 2-week operant conditioning of mice with standard or isocaloric highly palatable food. We investigated the behavioral effects and dendritic spine modifications in the NAc. We compared the translating mRNA in NAc neurons identified by the type of dopamine receptors they express, depending on the type of food and training. We tested the consequences of invalidation of an abundant downregulated gene, Ncdn (Neurochondrin).RESULTS:Operant conditioning for highly palatable food increases motivation for food even in well-fed mice. In control mice, free access to regular or highly palatable food results in increased weight as compared to regular food only. Highly palatable food increases spine density in the NAc. In animals trained for highly palatable food, translating mRNAs are modified in NAc dopamine D2-receptor-expressing neurons, mostly corresponding to striatal projection neurons, but not in those expressing D1-receptors. Knock-out of Ncdn, an abundant down-regulated gene, opposes the conditioning-induced changes in satiety-sensitive feeding behavior and apparent motivation for highly palatable food, suggesting down-regulation may be a compensatory mechanism.CONCLUSIONS:Our results emphasize the importance of mRNA alterations D2 striatal projection neurons in the NAc in the behavioral consequences of highly palatable food conditioning and suggest a modulatory contribution of Ncdn downregulation.

Astrocytes have been shown to play a central role in Alzheimer’s Disease (AD). However, the genes and biological pathways underlying disease manifestation are unknown, and it is unclear whether regional molecular differences among astrocytes contribute to regional specificity of disease. Here, we began to address these challenges with integrated experimental and computational approaches. We constructed a human astrocyte-specific functional gene network using Bayesian integration of a large compendium of human functional genomics data, as well as regional astrocyte gene expression profiles we generated in the mouse. This network identifies likely region-specific astrocyte pathways that operate in healthy brains. We leveraged our findings to compile genome-wide astrocyte-associated disease-gene predictions, employing a novel network-guided differential expression analysis (NetDIFF). We also used this data to predict a list of astrocyte-expressed genes mediating region-specific human disease, using a network-guided shortest path method (NetPATH). Both the network and our results are publicly available using an interactive web interface at . Our experimental and computational studies propose a strategy for disease gene and pathway prediction that may be applied to a host of human neurological disorders. ### Competing Interest Statement The authors have declared no competing interest.

Neuronal DNA modifications differ from those in other cells, including methylation outside CpG context and abundant 5-hydroxymethylation whose relevance for neuronal identities are unclear. Striatal projection neurons expressing D1 or D2 dopamine receptors allow addressing this question, as they share many characteristics but differ in their gene expression profiles, connections, and functional roles. We compare translating mRNAs and DNA modifications in these two populations. DNA methylation differences occur predominantly in large genomic clusters including differentially expressed genes, potentially important for D1 and D2 neurons. Decreased gene body methylation is associated with higher gene expression. Hydroxymethylation differences are more scattered and affect transcription factor binding sites, which can influence gene expression. We also find a strong genome-wide hydroxymethylation asymmetry between the two DNA strands, particularly pronounced at expressed genes and retrotransposons. These results identify novel properties of neuronal DNA modifications and unveil epigenetic characteristics of striatal projection neurons heterogeneity.

The cholinergic interneurons (ChATs) of the nucleus accumbens (NAc) have a critical role in the activity of this region, specifically in the context of major depressive disorder. To understand the circuitry regulating this behavior we sought to determine the areas that directly project to these cells/interneurons by utilizing the monosynaptic cell-specific tracing technique. Mapping showed monosynaptic projections that are exclusive to NAc ChATs. To determine if some of these projections are altered in a depression mouse model, we used mice that do not express the calcium binding protein p11 specifically in ChATs (ChAT-p11 cKO) and display a depressive-like phenotype. Our data demonstrated that while the overall projection areas remain similar between wild type and in ChAT-p11 cKO mice, the number of projections coming from the ventral hippocampus (vHIP) is significantly reduced in the ChAT-p11 cKO mice. Furthermore, using optogenetics and electrophysiology we showed that glutamatergic projections from vHIP to NAc ChATs are severely altered in mutant mice. These results show that specific alterations in the circuitry of the accumbal ChAT interneurons could play an important role in the regulation of depressive-like behavior, reward seeking behavior in addictions, or psychiatric symptoms in neurodegenerative diseases. ### Competing Interest Statement The authors have declared no competing interest.

Significance γ-secretase activating protein (GSAP) has emerged as a key regulator of γ-secretase. In cells, GSAP exists primarily in the form of a 16-kDa fragment known as GSAP-16K. In this study, we report the finding that GSAP-16K undergoes phase separation in vitro and in cells. Importantly, the outcome of GSAP-16K phase separation directly regulates the protease activity of human γ-secretase. Through direct interaction with the substrate amyloid precursor protein–C-terminal 99-residue fragment, GSAP-16K in dilute phase favors the production of β-amyloid peptide 42 (Aβ42) but not Aβ40. These observations not only explain how GSAP activates γ-secretase but also identify their interaction as a target of potential therapeutic intervention.

Forebrain dopamine-sensitive (dopaminoceptive) neurons play a key role in movement, action selection, motivation, and working memory. Their activity is altered in Parkinson's disease, addiction, schizophrenia, and other conditions, and drugs that stimulate or antagonize dopamine receptors have major therapeutic applications. Yet, similarities and differences between the various neuronal populations sensitive to dopamine have not been systematically explored. To characterize them, we compared translating mRNAs in the dorsal striatum and nucleus accumbens neurons expressing D1 or D2 dopamine receptor and prefrontal cortex neurons expressing D1 receptor. We identified genome-wide cortico-striatal, striatal D1/D2 and dorso/ventral differences in the translating mRNA and isoform landscapes, which characterize dopaminoceptive neuronal populations. Expression patterns and network analyses identified novel transcription factors with presumptive roles in these differences. Prostaglandin E2 (PGE2) was a candidate upstream regulator in the dorsal striatum. We pharmacologically explored this hypothesis and showed that misoprostol, a PGE2 receptor agonist, decreased the excitability of D2 striatal projection neurons in slices, and diminished their activity in vivo during novel environment exploration. We found that misoprostol also modulates mouse behavior including by facilitating reversal learning. Our study provides powerful resources for characterizing dopamine target neurons, new information about striatal gene expression patterns and regulation. It also reveals the unforeseen role of PGE2 in the striatum as a potential neuromodulator and an attractive therapeutic target.

Major depressive disorder (MDD) is a severe, common mood disorder. While reduced cerebrospinal fluid (CSF) flow adversely affects brain metabolism and fluid balance in the aging population and during development, only indirect evidence links aberrant CSF circulation with many diseases including neurological, neurodegenerative, and psychiatric disorders, such as anxiety and depression. Here we show a very high concentration of p11 as a key molecular determinant for depression in ependymal cells, which is significantly decreased in patients with MDD, and in two mouse models of depression induced by chronic stress, such as restraint and social isolation. The loss of p11 in ependymal cells causes disoriented ependymal planar cell polarity (PCP), reduced CSF flow, and depression-like and anxiety-like behaviors. p11 intrinsically controls PCP core genes, which mediates CSF flow. Viral expression of p11 in ependymal cells specifically rescues the pathophysiological and behavioral deficits caused by loss of p11. Taken together, our results identify a new role and a key molecular determinant for ependymal cell-driven CSF flow in mood disorders and suggest a novel strategy for development of treatments for stress-associated neurological, neurodegenerative, and psychiatric disorders.

Serotonin receptor 4 (5-HT 4 R) plays an important role in regulating mood, anxiety, and cognition, and drugs that activate this receptor have fast-acting antidepressant (AD)-like effects in preclinical models. However, 5-HT 4 R is widely expressed throughout the central nervous system (CNS) and periphery, making it difficult to pinpoint the cell types and circuits underlying its effects. Therefore, we generated a Cre-dependent 5-HT 4 R knockout mouse line to dissect the function of 5-HT 4 R in specific brain regions and cell types. We show that the loss of functional 5-HT 4 R specifically from excitatory neurons of hippocampus led to robust AD-like behavioral responses and an elevation in baseline anxiety. 5-HT 4 R was necessary to maintain the proper excitability of dentate gyrus (DG) granule cells and cell type-specific molecular profiling revealed a dysregulation of genes necessary for normal neural function and plasticity in cells lacking 5-HT 4 R. These adaptations were accompanied by an increase in the number of immature neurons in ventral, but not dorsal, dentate gyrus, indicating a broad impact of 5-HT 4 R loss on the local cellular environment. This study is the first to use conditional genetic targeting to demonstrate a direct role for hippocampal 5-HT 4 R signaling in modulating mood and anxiety. Our findings also underscore the need for cell type-based approaches to elucidate the complex action of neuromodulatory systems on distinct neural circuits.

SummaryMicroglia, the macrophages of the brain, are increasingly recognized to play a key role in synaptic plasticity and function; however, the underlying mechanisms remain elusive. Presenilin 1 (PS1) is an essential protein involved in learning and memory, through neuronal mechanisms. Loss of Presenilin function in neurons impairs synapse plasticity and causes cognitive deficits in mice. Surprisingly, here we show memory enhancement in mice by deleting PS1 selectively in microglia. We further demonstrate increased synapse transmission and in vivo neuronal activity in mice by depleting PS1 during microglial development, but not after microglial maturation. Remarkably, conditional deletion of PS1 in microglia during development increased memory retention in adulthood and was dependent on the NMDA receptor subunit GluN2B. In vivo calcium imaging of freely behaving mice revealed increased amplitude of neuronal Ca2+ transients in the CA1 hippocampus of PS1 cKO mice compared to control mice, suggesting a greater CA1 engagement during novel object exploration. Finally, loss of PS1 in microglia mitigated synaptic and cognitive deficits in a mouse model of Alzheimer’s disease. Together our results reveal a novel mechanism and function of PS1 in microglia in which modulation can enhance neuronal activity, learning and memory in mice.

Biochemical, pathogenic, and human genetic data confirm that GSAP (γ-secretase activating protein), a selective γ-secretase modulatory protein, plays important roles in Alzheimer’s disease (AD) and Down’s syndrome. However, the molecular mechanism(s) underlying GSAP-dependent pathogenesis remains largely elusive. Here, through unbiased proteomics and single-nuclei RNAseq, we identified that GSAP regulates multiple biological pathways, including protein phosphorylation, trafficking, lipid metabolism, and mitochondrial function. We demonstrated that GSAP physically interacts with the Fe65–APP complex to regulate APP trafficking/partitioning. GSAP is enriched in the mitochondria-associated membrane (MAM) and regulates lipid homeostasis through the amyloidogenic processing of APP. GSAP deletion generates a lipid environment unfavorable for AD pathogenesis, leading to improved mitochondrial function and the rescue of cognitive deficits in an AD mouse model. Finally, we identified a novel GSAP single-nucleotide polymorphism that regulates its brain transcript level and is associated with an increased AD risk. Together, our findings indicate that GSAP impairs mitochondrial function through its MAM localization and that lowering GSAP expression reduces pathological effects associated with AD.

Microglial cells play a key role in brain homeostasis from development to adulthood. Here we show the involvement of a site-specific phosphorylation of Presenilin 1 (PS1) in microglial development. Profiles of microglia-specific transcripts in different temporal stages of development, combined with multiple systematic transcriptomic analysis and quantitative determination of microglia progenitors, indicate that the phosphorylation of PS1 at serine 367 is involved in the temporal dynamics of microglial development, specifically in the developing brain rudiment during embryonic microgliogenesis. We constructed a developing brain-specific microglial network to identify transcription factors linked to PS1 during development. Our data showed that PS1 functional connections appear through interaction hubs at Pu.1, Irf8 and Rela-p65 transcription factors. Finally, we showed that the total number of microglia progenitors was markedly reduced in the developing brain rudiment of embryos lacking PS1 phosphorylation compared to WT. Our work identifies a novel role for PS1 in microglial development.

The delayed behavioral response to chronic antidepressants depends on dynamic changes in the hippocampus. It was suggested that the antidepressant protein p11 and the chromatin remodeling factor SMARCA3 mediate this delayed response by inducing transcriptional changes in hippocampal neurons. However, what target genes are regulated by the p11/SMARCA3 complex to mediate the behavioral response to antidepressants, and what cell type mediates these molecular changes remain unknown. Here we report that the p11/SMARCA3 complex represses Neurensin-2 transcription in hippocampal parvalbumin-expressing interneurons after chronic treatment with Selective Serotonin Reuptake Inhibitors (SSRI). The behavioral response to antidepressants requires upregulation of p11, accumulation of SMARCA3 in the cell nucleus, and a consequent repression of Neurensin-2 transcription in these interneurons. We elucidate a functional role for p11/SMARCA3/Neurensin-2 pathway in regulating AMPA-receptor signaling in parvalbumin-expressing interneurons, a function that is enhanced by chronic treatment with SSRIs. These results link SSRIs to dynamic glutamatergic changes and implicate p11/SMARCA3/Neurensin-2 pathway in the development of more specific and efficient therapeutic strategies for neuropsychiatric disorders.

Among the hallmarks of major depressive disorders (MDD) are molecular, functional, and morphological impairments in the hippocampus. Recent studies suggested a key role for hippocampal GABAergic interneurons both in depression and in the response to its treatments. These interneurons highly express the chromatin-remodeler SMARCA3 which mediates the response to chronic antidepressants in an unknown mechanism. Using cell-type-specific molecular and physiological approaches, we report that SMARCA3 mediates the glutamatergic signaling in interneurons by repressing the expression of the neuronal protein, Neurensin-2. This vesicular protein associates with endosomes and postsynaptic proteins and is highly and selectively expressed in subpopulations of GABAergic interneurons. Upregulation of Neurensin-2 in the hippocampus either by stress, viral overexpression, or by SMARCA3 deletion, results in depressive-like behaviors. In contrast, the deletion of Neurensin-2 confers resilience to stress and induces AMPA receptor localization to synapses. This pathway which bidirectionally affects emotional behavior could be involved in neuropsychiatric disorders, and suggests novel therapeutic approaches.

Major Depressive Disorder is strongly associated with glutamatergic dysfunctions, whereas the most prescribed antidepressant treatments enhance monoamine availability. However, these antidepressants are only partially effective and require chronic treatment to achieve their effects. Chronic treatment with antidepressants leads to elevated levels of the protein p11 in the brain, primarily the cortex and the hippocampus. Still, the cellular functions that are regulated by p11 following chronic antidepressants as well as the brain regions and the neuronal cell types involved remain unknown.

A correction to this paper has been published and can be accessed via a link at the top of the paper.

Amyloid-beta peptides generated by β-secretase- and γ-secretase-mediated successive cleavage of amyloid precursor protein are believed to play a causative role in Alzheimer's disease. Thus, reducing amyloid-beta generation by modulating γ-secretase remains a promising approach for Alzheimer's disease therapeutic development. Here, we screened fruit extracts of Ligustrum lucidum Ait. (Oleaceae) and identified active fractions that increase the C-terminal fragment of amyloid precursor protein and reduce amyloid-beta production in a neuronal cell line. These fractions contain a mixture of two isomeric pentacyclic triterpene natural products, 3-O-cis- or 3-O-trans-p-coumaroyl maslinic acid (OCMA), in different ratios. We further demonstrated that trans-OCMA specifically inhibits γ-secretase and decreases amyloid-beta levels without influencing cleavage of Notch. By using photoactivatable probes targeting the subsites residing in the γ-secretase active site, we demonstrated that trans-OCMA selectively affects the S1 subsite of the active site in this protease. Treatment of Alzheimer's disease transgenic model mice with trans-OCMA or an analogous carbamate derivative of a related pentacyclic triterpene natural product, oleanolic acid, rescued the impairment of synaptic plasticity. This work indicates that the naturally occurring compound trans-OCMA and its analogues could become a promising class of small molecules for Alzheimer's disease treatment.

Selective serotonin reuptake inhibitors (SSRIs) are the most widely prescribed drugs for mood disorders. While the mechanism of SSRI action is still unknown, SSRIs are thought to exert therapeutic effects by elevating extracellular serotonin levels in the brain, and remodel the structural and functional alterations dysregulated during depression. To determine their precise mode of action, we tested whether such neuroadaptive processes are modulated by regulation of specific gene expression programs. Here we identify a transcriptional program regulated by activator protein-1 (AP-1) complex, formed by c-Fos and c-Jun that is selectively activated prior to the onset of the chronic SSRI response. The AP-1 transcriptional program modulates the expression of key neuronal remodeling genes, including S100a10 (p11), linking neuronal plasticity to the antidepressant response. We find that AP-1 function is required for the antidepressant effect in vivo. Furthermore, we demonstrate how neurochemical pathways of BDNF and FGF2, through the MAPK, PI3K, and JNK cascades, regulate AP-1 function to mediate the beneficial effects of the antidepressant response. Here we put forth a sequential molecular network to track the antidepressant response and provide a new avenue that could be used to accelerate or potentiate antidepressant responses by triggering neuroplasticity.

Glutamatergic mossy cells (MCs) are responsible for the associational and commissural connectivity of the dentate gyrus. MCs are widely distributed along the dorsoventral axis, but potential heterogeneity within MCs is scarcely explored. Here, we showed that MCs consist of two subpopulations which differ in their neuronal properties and functions. We discovered that MCs, depending on their dorsoventral location, extend distinct axonal projections in the molecular layers. Comparative transcriptional profiling of dorsal and ventral MCs revealed different neurobiological characteristics in axon guidance and synapse assembly. Despite common activation by external stimuli, dorsal MCs, but not ventral MCs, provide net inhibitory control on granule cells across the longitudinal axis. Furthermore, dorsal MC inhibition, unlikely that of ventral MCs, increases behavioral anxiety and disables rapid contextual discrimination. Collectively, dorsoventral heterogeneity of MCs may provide a novel mechanism for functional differentiation as well as distinct association along the longitudinal extent of the hippocampus.

n amendment to this paper has been published and can be accessed via a link at the top of the paper.