FACS-array profiling of striatal projection neuron subtypes in juvenile and adult mouse brains

Msh3 and Pms1 Set Neuronal CAG-repeat Migration Rate to Drive Selective Striatal and Cortical Pathogenesis in HD Mice

Modifiers of Huntington's disease (HD) include mismatch repair (MMR) genes; however, their underlying disease-altering mechanisms remain unresolved. Knockout (KO) alleles for 9 HD GWAS modifiers/MMR genes were crossed to the Q140 Huntingtin (mHtt) knock-in mice to probe such mechanisms. Four KO mice strongly ( Msh3 and Pms1 ) or moderately ( Msh2 and Mlh1 ) rescue a triad of adult-onset, striatal medium-spiny-neuron (MSN)-selective phenotypes: somatic Htt DNA CAG-repeat expansion, transcriptionopathy, and mHtt protein aggregation. Comparatively, Q140 cortex also exhibits an analogous, but later-onset, pathogenic triad that is Msh3 -dependent. Remarkably, Q140/homozygous Msh3-KO lacks visible mHtt aggregates in the brain, even at advanced ages (20-months). Moreover, Msh3 -deficiency prevents striatal synaptic marker loss, astrogliosis, and locomotor impairment in HD mice. Purified Q140 MSN nuclei exhibit highly linear age-dependent mHtt DNA repeat expansion (i.e. repeat migration), with modal-CAG increasing at +8.8 repeats/month (R 2 =0.98). This linear rate is reduced to 2.3 and 0.3 repeats/month in Q140 with Msh3 heterozygous and homozygous alleles, respectively. Our study defines somatic Htt CAG-repeat thresholds below which there are no detectable mHtt nuclear or neuropil aggregates. Mild transcriptionopathy can still occur in Q140 mice with stabilized Htt 140-CAG repeats, but the majority of transcriptomic changes are due to somatic repeat expansion. Our analysis reveals 479 genes with expression levels highly correlated with modal-CAG length in MSNs. Thus, our study mechanistically connects HD GWAS genes to selective neuronal vulnerability in HD, in which Msh3 and Pms1 set the linear rate of neuronal mHtt CAG-repeat migration to drive repeat-length dependent pathogenesis; and provides a preclinical platform for targeting these genes for HD suppression across brain regions.

One Sentence Summary

Msh3 and Pms1 are genetic drivers of sequential striatal and cortical pathogenesis in Q140 mice by mediating selective CAG-repeat migration in HD vulnerable neurons.

Neurochemistry and circuit organization of the lateral spiriform nucleus of birds: A uniquely nonmammalian direct pathway component of the basal ganglia

We used diverse methods to characterize the role of avian lateral spiriform nucleus (SpL) in basal ganglia motor function. Connectivity analysis showed that SpL receives input from globus pallidus (GP), and the intrapeduncular nucleus (INP) located ventromedial to GP, whose neurons express numerous striatal markers. SpL-projecting GP neurons were large and aspiny, while SpL-projecting INP neurons were medium sized and spiny. Connectivity analysis further showed that SpL receives inputs from subthalamic nucleus (STN) and substantia nigra pars reticulata (SNr), and that the SNr also receives inputs from GP, INP, and STN. Neurochemical analysis showed that SpL neurons express ENK, GAD, and a variety of pallidal neuron markers, and receive GABAergic terminals, some of which also contain DARPP32, consistent with GP pallidal and INP striatal inputs. Connectivity and neurochemical analysis showed that the SpL input to tectum prominently ends on GABAA receptor-enriched tectobulbar neurons. Behavioral studies showed that lesions of SpL impair visuomotor behaviors involving tracking and pecking moving targets. Our results suggest that SpL modulates brainstem-projecting tectobulbar neurons in a manner comparable to the demonstrated influence of GP internus on motor thalamus and of SNr on tectobulbar neurons in mammals. Given published data in amphibians and reptiles, it seems likely the SpL circuit represents a major direct pathway-type circuit by which the basal ganglia exerts its motor influence in nonmammalian tetrapods. The present studies also show that avian striatum is divided into three spatially segregated territories with differing connectivity, a medial striato-nigral territory, a dorsolateral striato-GP territory, and the ventrolateral INP motor territory.

Repulsive Sema3E-Plexin-D1 signaling coordinates both axonal extension and steering via activating an autoregulatory factor, Mtss1

Axon guidance molecules are critical for neuronal pathfinding because they regulate directionality and growth pace during nervous system development. However, the molecular mechanisms coordinating proper axonal extension and turning are poorly understood. Here, metastasis suppressor 1 (Mtss1), a membrane protrusion protein, ensured axonal extension while sensitizing axons to the Semaphorin 3E (Sema3E)-Plexin-D1 repulsive cue. Sema3E-Plexin-D1 signaling enhanced Mtss1 expression in projecting striatonigral neurons. Mtss1 localized to the neurite axonal side and regulated neurite outgrowth in cultured neurons. Mtss1 also aided Plexin-D1 trafficking to the growth cone, where it signaled a repulsive cue to Sema3E. Mtss1 ablation reduced neurite extension and growth cone collapse in cultured neurons. Mtss1-knockout mice exhibited fewer striatonigral projections and irregular axonal routes, and these defects were recapitulated in Plxnd1- or Sema3e-knockout mice. These findings demonstrate that repulsive axon guidance activates an exquisite autoregulatory program coordinating both axonal extension and steering during neuronal pathfinding.

Balance Between Projecting Neuronal Populations of the Nucleus Accumbens Controls Social Behavior in Mice

BACKGROUND

Deficient social interactions are a hallmark of major neuropsychiatric disorders, and accumulating evidence points to altered social reward and motivation as key underlying mechanisms of these pathologies. In the present study, we further explored the role of the balance of activity between D1 and D2 receptor-expressing striatal projection neurons (D1R- and D2R-SPNs) in the control of social behavior, challenging the hypothesis that excessive D2R-SPN activity, rather than deficient D1R-SPN activity, compromises social behavior.

METHODS

We selectively ablated D1R- and D2R-SPNs using an inducible diphtheria toxin receptor-mediated cell targeting strategy and assessed social behavior as well as repetitive/perseverative behavior, motor function, and anxiety levels. We tested the effects of optogenetic stimulation of D2R-SPNs in the nucleus accumbens (NAc) and pharmacological compounds repressing D2R-SPN.

RESULTS

Targeted deletion of D1R-SPNs in the NAc blunted social behavior in mice, facilitated motor skill learning, and increased anxiety levels. These behaviors were normalized by pharmacological inhibition of D2R-SPN, which also repressed transcription in the efferent nucleus, the ventral pallidum. Ablation of D1R-SPNs in the dorsal striatum had no impact on social behavior but impaired motor skill learning and decreased anxiety levels. Deletion of D2R-SPNs in the NAc produced motor stereotypies but facilitated social behavior and impaired motor skill learning. We mimicked excessive D2R-SPN activity by optically stimulating D2R-SPNs in the NAc and observed a severe deficit in social interaction that was prevented by D2R-SPN pharmacological inhibition.

CONCLUSIONS

Repressing D2R-SPN activity may represent a promising therapeutic strategy to relieve social deficits in neuropsychiatric disorders.

Mitofusin-2 in nucleus accumbens D2-MSNs regulates social dominance and neuronal function

The nucleus accumbens (NAc) is a brain hub regulating motivated behaviors, including social competitiveness. Mitochondrial function in the NAc links anxiety with social competitiveness, and the mitochondrial fusion protein mitofusin 2 (Mfn2) in NAc neurons regulates anxiety-related behaviors. However, it remains unexplored whether accumbal Mfn2 levels also affect social behavior and whether Mfn2 actions in the emotional and social domain are driven by distinct cell types. Here, we found that subordinate-prone highly anxious rats show decreased accumbal Mfn2 levels and that Mfn2 overexpression promotes dominant behavior. In mice, selective Mfn2 downregulation in NAc dopamine D2 receptor-expressing medium spiny neurons (D2-MSNs) induced social subordination, accompanied by decreased accumbal mitochondrial functions and decreased excitability in D2-MSNs. Instead, D1-MSN-targeted Mfn2 downregulation affected competitive ability only transiently and likely because of an increase in anxiety-like behaviors. Our results assign dissociable cell-type specific roles to Mfn2 in the NAc in modulating social dominance and anxiety.

Involvement of brain cell phenotypes in stress-vulnerability and resilience

Stress-related disorders' prevalence is epidemically increasing in modern society, leading to a severe impact on individuals' well-being and a great economic burden on public resources. Based on this, it is critical to understand the mechanisms by which stress induces these disorders. The study of stress made great progress in the past decades, from deeper into the hypothalamic-pituitary-adrenal axis to the understanding of the involvement of a single cell subtype on stress outcomes. In fact, many studies have used state-of-the-art tools such as chemogenetic, optogenetic, genetic manipulation, electrophysiology, pharmacology, and immunohistochemistry to investigate the role of specific cell subtypes in the stress response. In this review, we aim to gather studies addressing the involvement of specific brain cell subtypes in stress-related responses, exploring possible mechanisms associated with stress vulnerability versus resilience in preclinical models. We particularly focus on the involvement of the astrocytes, microglia, medium spiny neurons, parvalbumin neurons, pyramidal neurons, serotonergic neurons, and interneurons of different brain areas in stress-induced outcomes, resilience, and vulnerability to stress. We believe that this review can shed light on how diverse molecular mechanisms, involving specific receptors, neurotrophic factors, epigenetic enzymes, and miRNAs, among others, within these brain cell subtypes, are associated with the expression of a stress-susceptible or resilient phenotype, advancing the understanding/knowledge on the specific machinery implicate in those events.

Transcriptional control of nucleus accumbens neuronal excitability by retinoid X receptor alpha tunes sensitivity to drug rewards

The complex nature of the transcriptional networks underlying addictive behaviors suggests intricate cooperation between diverse gene regulation mechanisms that go beyond canonical activity-dependent pathways. Here, we implicate in this process a nuclear receptor transcription factor, retinoid X receptor alpha (RXRα), which we initially identified bioinformatically as associated with addiction-like behaviors. In the nucleus accumbens (NAc) of male and female mice, we show that although its own expression remains unaltered after cocaine exposure, RXRα controls plasticity- and addiction-relevant transcriptional programs in both dopamine receptor D1- and D2-expressing medium spiny neurons, which in turn modulate intrinsic excitability and synaptic activity of these NAc cell types. Behaviorally, bidirectional viral and pharmacological manipulation of RXRα regulates drug reward sensitivity in both non-operant and operant paradigms. Together, this study demonstrates a key role for NAc RXRα in promoting drug addiction and paves the way for future studies of rexinoid signaling in psychiatric disease states.

A Review of Research on the Association between Neuron-Astrocyte Signaling Processes and Depressive Symptoms

Depression is a mental illness that has a serious negative impact on physical and mental health. The pathophysiology of depression is still unknown, and therapeutic medications have drawbacks, such as poor effectiveness, strong dependence, adverse drug withdrawal symptoms, and harmful side effects. Therefore, the primary purpose of contemporary research is to understand the exact pathophysiology of depression. The connection between astrocytes, neurons, and their interactions with depression has recently become the focus of great research interest. This review summarizes the pathological changes of neurons and astrocytes, and their interactions in depression, including the alterations of mid-spiny neurons and pyramidal neurons, the alterations of astrocyte-related biomarkers, and the alterations of gliotransmitters between astrocytes and neurons. In addition to providing the subjects of this research and suggestions for the pathogenesis and treatment techniques of depression, the intention of this article is to more clearly identify links between neuronal-astrocyte signaling processes and depressive symptoms.

Inducible CRISPR Epigenome Systems Mimic Cocaine Induced Bidirectional Regulation of Nab2 and Egr3

Substance use disorder is a chronic disease and a leading cause of disability around the world. The NAc is a major brain hub mediating reward behavior. Studies demonstrate exposure to cocaine is associated with molecular and functional imbalance in NAc medium spiny neuron subtypes (MSNs), dopamine receptor 1 and 2 enriched D1-MSNs and D2-MSNs. We previously reported repeated cocaine exposure induced transcription factor early growth response 3 (Egr3) mRNA in NAc D1-MSNs, and reduced it in D2-MSNs. Here, we report our findings of repeated cocaine exposure in male mice inducing MSN subtype-specific bidirectional expression of the Egr3 corepressor NGFI-A-binding protein 2 (Nab2). Using CRISPR activation and interference (CRISPRa and CRISPRi) tools combined with Nab2 or Egr3-targeted sgRNAs, we mimicked these bidirectional changes in Neuro2a cells. Furthermore, we investigated D1-MSN- and D2-MSN-specific expressional changes of histone lysine demethylases Kdm1a, Kdm6a, and Kdm5c in NAc after repeated cocaine exposure in male mice. Since Kdm1a showed bidirectional expression patterns in D1-MSNs and D2-MSNs, like Egr3, we developed a light-inducible Opto-CRISPR-KDM1a system. We were able to downregulate Egr3 and Nab2 transcripts in Neuro2A cells and cause similar bidirectional expression changes we observed in D1-MSNs and D2-MSNs of mouse repeated cocaine exposure model. Contrastingly, our Opto-CRISPR-p300 activation system induced the Egr3 and Nab2 transcripts and caused opposite bidirectional transcription regulations. Our study sheds light on the expression patterns of Nab2 and Egr3 in specific NAc MSNs in cocaine action and uses CRISPR tools to further mimic these expression patterns.SIGNIFICANCE STATEMENT Substance use disorder is a major societal issue. The lack of medication to treat cocaine addiction desperately calls for a treatment development based on precise understanding of molecular mechanisms underlying cocaine addiction. In this study, we show that Egr3 and Nab2 are bidirectionally regulated in mouse NAc D1-MSNs and D2-MSNs after repeated exposure to cocaine. Furthermore, histone lysine demethylations enzymes with putative EGR3 binding sites showed bidirectional regulation in D1- and D2-MSNs after repeated exposure to cocaine. Using Cre- and light-inducible CRISPR tools, we show that we can mimic this bidirectional regulation of Egr3 and Nab2 in Neuro2a cells.

CREB Binding at the Zfp189 Promoter Within Medium Spiny Neuron Subtypes Differentially Regulates Behavioral and Physiological Adaptations Over the Course of Cocaine Use

BACKGROUND

Over the course of chronic drug use, brain transcriptional neuroadaptation is thought to contribute to a change in drug use behavior over time. The function of the transcription factor CREB (cAMP response element binding protein) within the nucleus accumbens (NAc) has been well documented in opposing the rewarding properties of many classes of drugs, yet the gene targets through which CREB causally manifests these lasting neuroadaptations remain unknown. Here, we identify zinc finger protein 189 (Zfp189) as a CREB target gene that is transcriptionally responsive to acute and chronic cocaine use within the NAc of mice.

METHODS

To investigate the role of the CREB-Zfp189 interaction in cocaine use, we virally delivered modified clustered regularly interspaced short palindromic repeats (CRISPR)/dCas9 constructs capable of selectively localizing CREB to the Zfp189 gene promoter in the NAc of mice.

RESULTS

We observed that CREB binding to the Zfp189 promoter increased Zfp189 expression and diminished the reinforcing responses to cocaine. Furthermore, we showed that NAc Zfp189 expression increased within D1 medium spiny neurons in response to acute cocaine but increased in both D1- and D2-expressing medium spiny neurons in response to chronic cocaine. CREB-mediated induction of Zfp189 potentiated electrophysiological activity of D1- and D2-expressing medium spiny neurons, recapitulating the known effect of CREB on these neurons. Finally, targeting CREB to the Zfp189 promoter within NAc Drd2-expressing neurons, but not Drd1-expressing neurons, was sufficient to diminish cocaine-conditioned behaviors.

CONCLUSIONS

Together, these findings point to the CREB-Zfp189 interaction within the NAc Drd2+ neurons as a molecular signature of chronic cocaine use that is causal in counteracting the reinforcing effects of cocaine.

Cell-type specific profiling of histone post-translational modifications in the adult mouse striatum

Epigenetic gene regulation in the heterogeneous brain remains challenging to decipher with current strategies. Bulk tissue analysis from pooled subjects reflects the average of cell-type specific changes across cell-types and individuals, which obscures causal relationships between epigenetic modifications, regulation of gene expression, and complex pathology. To address these limitations, we optimized a hybrid protocol, ICuRuS, for the isolation of nuclei tagged in specific cell-types and histone post translational modification profiling from the striatum of a single mouse. We combined affinity-based isolation of the medium spiny neuron subtypes, Adenosine 2a Receptor or Dopamine Receptor D1, with cleavage of histone-DNA complexes using an antibody-targeted micrococcal nuclease to release DNA complexes for paired end sequencing. Unlike fluorescence activated cell sorting paired with chromatin immunoprecipitation, ICuRuS allowed for robust epigenetic profiling at cell-type specific resolution. Our analysis provides a framework to understand combinatorial relationships between neuronal-subtype-specific epigenetic modifications and gene expression.

DNA methylation and hydroxymethylation characterize the identity of D1 and D2 striatal projection neurons

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.

Mapping brain gene coexpression in daytime transcriptomes unveils diurnal molecular networks and deciphers perturbation gene signatures

Brain tissue transcriptomes may be organized into gene coexpression networks, but their underlying biological drivers remain incompletely understood. Here, we undertook a large-scale transcriptomic study using 508 wild-type mouse striatal tissue samples dissected exclusively in the afternoons to define 38 highly reproducible gene coexpression modules. We found that 13 and 11 modules are enriched in cell-type and molecular complex markers, respectively. Importantly, 18 modules are highly enriched in daily rhythmically expressed genes that peak or trough with distinct temporal kinetics, revealing the underlying biology of striatal diurnal gene networks. Moreover, the diurnal coexpression networks are a dominant feature of daytime transcriptomes in the mouse cortex. We next employed the striatal coexpression modules to decipher the striatal transcriptomic signatures from Huntington's disease models and heterozygous null mice for 52 genes, uncovering novel functions for Prkcq and Kdm4b in oligodendrocyte differentiation and bipolar disorder-associated Trank1 in regulating anxiety-like behaviors and nocturnal locomotion.

Cocaine regulation of Nr4a1 chromatin bivalency and mRNA in male and female mice

Cocaine epigenetically regulates gene expression via changes in histone post-translational modifications (HPTMs). We previously found that the immediate early gene Nr4a1 is epigenetically activated by cocaine in mouse brain reward regions. However, few studies have examined multiple HPTMs at a single gene. Bivalent gene promoters are simultaneously enriched in both activating (H3K4me3 (K4)) and repressive (H3K27me3 (K27)) HPTMs. As such, bivalent genes are lowly expressed but poised for activity-dependent gene regulation. In this study, we identified K4&K27 bivalency at Nr4a1 following investigator-administered cocaine in male and female mice. We applied sequential chromatin immunoprecipitation and qPCR to define Nr4a1 bivalency and expression in striatum (STR), prefrontal cortex (PFC), and hippocampus (HPC). We used Pearson's correlation to quantify relationships within each brain region across treatment conditions for each sex. In female STR, cocaine increased Nr4a1 mRNA while maintaining Nr4a1 K4&K27 bivalency. In male STR, cocaine enriched repressive H3K27me3 and K4&K27 bivalency at Nr4a1 and maintained Nr4a1 mRNA. Furthermore, cocaine epigenetically regulated a putative NR4A1 target, Cartpt, in male PFC. This study defined the epigenetic regulation of Nr4a1 in reward brain regions in male and female mice following cocaine, and, thus, shed light on the biological relevance of sex to cocaine use disorder.

Fluorescence quenching by high-power LEDs for highly sensitive fluorescence in situ hybridization

Recent technical advances have made fluorescent in situ hybridization (ISH) a pivotal method to analyze neural tissue. In a highly sensitive ISH, it is important to reduce tissue autofluorescence. We developed a photobleaching device using a light-emitting diode (LED) illuminator to quench autofluorescence in neural tissue. This device was equipped with 12 high-power LEDs (30 W per single LED) and an evaporative cooling system, and these features achieved highly efficient bleaching of autofluorescence and minimized tissue damage. Even after 60 min of photobleaching with evaporative cooling, the temperature gain of the tissue slide was suppressed almost completely. The autofluorescence of lipofuscin-like granules completely disappeared after 60 min of photobleaching, as did other background autofluorescence observed in the mouse cortex and hippocampus. In combination with the recently developed fluorescent ISH method using the hybridization chain reaction (HCR), high signal/noise ratio imaging was achieved without reduction of ISH sensitivity to visualize rare mRNA at single copy resolution by quenching autofluorescence. Photobleaching by the LED illuminator was also effective in quenching the fluorescent staining of ISH-HCR. We performed multiround ISH by repeating the cycle of HCR staining, confocal imaging, and photobleaching. In addition to the two-round ISH, fluorescent immunohistochemistry or fluorescent Nissl staining was conducted on the same tissue. This LED illuminator provides a quick and simple way to reduce autofluorescence and quench fluorescent dyes for multiround ISH with minimum tissue degradation.

The transcription factor Zfp503 promotes the D1 MSN identity and represses the D2 MSN identity

The striatum is primarily composed of two types of medium spiny neurons (MSNs) expressing either D1- or D2-type dopamine receptors. However, the fate determination of these two types of neurons is not fully understood. Here, we found that D1 MSNs undergo fate switching to D2 MSNs in the absence of Zfp503. Furthermore, scRNA-seq revealed that the transcription factor Zfp503 affects the differentiation of these progenitor cells in the lateral ganglionic eminence (LGE). More importantly, we found that the transcription factors Sp8/9, which are required for the differentiation of D2 MSNs, are repressed by Zfp503. Finally, sustained Zfp503 expression in LGE progenitor cells promoted the D1 MSN identity and repressed the D2 MSN identity. Overall, our findings indicated that Zfp503 promotes the D1 MSN identity and represses the D2 MSN identity by regulating Sp8/9 expression during striatal MSN development.

Transcription factor Sp9 is a negative regulator of D1-type MSN development

The striatum is the main input structure of the basal ganglia, receiving information from the cortex and the thalamus and consisting of D1- and D2- medium spiny neurons (MSNs). D1-MSNs and D2-MSNs are essential for motor control and cognitive behaviors and have implications in Parkinson’s Disease. In the present study, we demonstrated that Sp9-positive progenitors produced both D1-MSNs and D2-MSNs and that Sp9 expression was rapidly downregulated in postmitotic D1-MSNs. Furthermore, we found that sustained Sp9 expression in lateral ganglionic eminence (LGE) progenitor cells and their descendants led to promoting D2-MSN identity and repressing D1-MSN identity during striatal development. As a result, sustained Sp9 expression resulted in an imbalance between D1-MSNs and D2-MSNs in the mouse striatum. In addition, the fate-changed D2-like MSNs survived normally in adulthood. Taken together, our findings supported that Sp9 was sufficient to promote D2-MSN identity and repress D1-MSN identity, and Sp9 was a negative regulator of D1-MSN fate.

The BDNF-TrkB Pathway Acts Through Nucleus Accumbens D2 Expressing Neurons to Mediate Stress Susceptible Outcomes

Brain-derived neurotrophic factor (BDNF) has a critical role in stress response including neuropsychiatric disorders that are precipitated by stress, such as major depressive disorder (MDD). BDNF acts through its full-length BDNF receptor tyrosine kinase B (TrkB) to trigger a pro-plasticity effect. In contrast, the truncated isoform of the BDNF receptor (TrkB.t1) triggers an anti-plasticity effect. In stress outcomes, BDNF acting in the hippocampus has a stress resilience effect, and, inversely, in the nucleus accumbens (NAc), BDNF acts as a stress susceptible molecule. It is unknown if BDNF-TrkB acts on a specific NAc projection neuron, i.e., medium spiny neuron (MSN or spiny projection neuron), a subtype in stress outcomes. To determine this, we performed chronic social or vicarious witness defeat stress (CSDS or CWDS) in mice expressing TrkB.t1 in dopamine receptor 1 or 2 containing MSNs (D1- or D2-MSNs). Our results showed that TrkB.t1 overexpression in NAc D2-MSNs prevented the CSDS-induced social avoidance or other stress susceptible behaviors in male and female mice. We further showed that this overexpression in D2-MSNs blocked stress susceptible behavior induced by intra-NAc BDNF infusion. In contrast, our results demonstrate that overexpression of TrkB.t1 on NAc D1-MSNs facilitates the SDS susceptible behaviors. Our study provides enhanced details into the NAc cell subtype role of BDNF-TrkB signaling in stress outcomes.

Translational profiling of mouse dopaminoceptive neurons reveals region-specific gene expression, exon usage, and striatal prostaglandin E2 modulatory effects

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.

Optimized Protocol for Proportionate CNS Cell Retrieval as a Versatile Platform for Cellular and Molecular Phenomapping in Aging and Neurodegeneration

Efficient purification of viable neural cells from the mature CNS has been historically challenging due to the heterogeneity of the inherent cell populations. Moreover, changes in cellular interconnections, membrane lipid and cholesterol compositions, compartment-specific biophysical properties, and intercellular space constituents demand technical adjustments for cell isolation at different stages of maturation and aging. Though such obstacles are addressed and partially overcome for embryonic premature and mature CNS tissues, procedural adaptations to an aged, progeroid, and degenerative CNS environment are underrepresented. Here, we describe a practical workflow for the acquisition and phenomapping of CNS neural cells at states of health, physiological and precocious aging, and genetically provoked neurodegeneration. Following recent, unprecedented evidence of post-mitotic cellular senescence (PoMiCS), the protocol appears suitable for such de novo characterization and phenotypic opposition to classical senescence. Technically, the protocol is rapid, efficient as for cellular yield and well preserves physiological cell proportions. It is suitable for a variety of downstream applications aiming at cell type-specific interrogations, including cell culture systems, Flow-FISH, flow cytometry/FACS, senescence studies, and retrieval of omic-scale DNA, RNA, and protein profiles. We expect suitability for transfer to other CNS targets and to a broad spectrum of engineered systems addressing aging, neurodegeneration, progeria, and senescence.

Dlx1/2-dependent expression of Meis2 promotes neuronal fate determination in the mammalian striatum

The striatum is a central regulator of behavior and motor function through the actions of D1 and D2 medium-sized spiny neurons (MSNs), which arise from a common lateral ganglionic eminence (LGE) progenitor. The molecular mechanisms of cell fate specification of these two neuronal subtypes are incompletely understood. Here, we found that deletion of murine Meis2, which is highly expressed in the LGE and derivatives, led to a large reduction in striatal MSNs due to a block in their differentiation. Meis2 directly binds to the Zfp503 and Six3 promoters and is required for their expression and specification of D1 and D2 MSNs, respectively. Finally, Meis2 expression is regulated by Dlx1/2 at least partially through the enhancer hs599 in the LGE subventricular zone. Overall, our findings define a pathway in the LGE whereby Dlx1/2 drives expression of Meis2, which subsequently promotes the fate determination of striatal D1 and D2 MSNs via Zfp503 and Six3.

Whole Genome DNA Methylation Profiling of D2 Medium Spiny Neurons in Mouse Nucleus Accumbens Using Two Independent Library Preparation Methods

DNA methylation plays essential roles in various cellular processes. Next-generation sequencing has enabled us to study the functional implication of DNA methylation across the whole genome. However, this approach usually requires a substantial amount of genomic DNA, which limits its application to defined cell types within a discrete brain region. Here, we applied two separate protocols, Accel-NGS Methyl-Seq (AM-seq) and Enzymatic Methyl-seq (EM-seq), to profile the methylome of D2 dopamine receptor-expressing medium spiny neurons (D2-MSNs) in mouse nucleus accumbens (NAc). Using 40 ng DNA extracted from FACS-isolated D2-MSNs, we found that both methods yielded comparably high-quality methylome data. Additionally, we identified numerous unmethylated regions (UMRs) as cell type-specific regulatory regions. By comparing the NAc D2-MSN methylome with the published methylomes of mouse prefrontal cortex excitatory neurons and neural progenitor cells (NPCs), we identified numerous differentially methylated CpG and non-CpG regions. Our study not only presents a comparison of these two low-input DNA whole genome methylation profiling protocols, but also provides a resource of DNA methylome of mouse accumbal D2-MSNs, a neuron type that has critical roles in addiction and other neuropsychiatric disorders.

A distinct D1-MSN subpopulation down-regulates dopamine to promote negative emotional state

Dopamine (DA) level in the nucleus accumbens (NAc) is critical for reward and aversion encoding. DA released from the ventral mesencephalon (VM) DAergic neurons increases the excitability of VM-projecting D1-dopamine receptor-expressing medium spiny neurons (D1-MSNs) in the NAc to enhance DA release and augment rewards. However, how such a DA positive feedback loop is regulated to maintain DA homeostasis and reward-aversion balance remains elusive. Here we report that the ventral pallidum (VP) projection of NAc D1-MSNs (D1^NAc-VP) is inhibited by rewarding stimuli and activated by aversive stimuli. In contrast to the VM projection of D1-MSN (D1^NAc-VM), activation of D1^NAc-VP projection induces aversion, but not reward. D1^NAc-VP MSNs are distinct from the D1^NAc-VM MSNs, which exhibit conventional functions of D1-MSNs. Activation of D1^NAc-VP projection stimulates VM GABAergic transmission, inhibits VM DAergic neurons, and reduces DA release into the NAc. Thus, D1^NAc-VP and D1^NAc-VM MSNs cooperatively control NAc dopamine balance and reward-aversion states.

DNA Epigenetics in Addiction Susceptibility

Addiction is a chronically relapsing neuropsychiatric disease that occurs in some, but not all, individuals who use substances of abuse. Relatively little is known about the mechanisms which contribute to individual differences in susceptibility to addiction. Neural gene expression regulation underlies the pathogenesis of addiction, which is mediated by epigenetic mechanisms, such as DNA modifications. A growing body of work has demonstrated distinct DNA epigenetic signatures in brain reward regions that may be associated with addiction susceptibility. Furthermore, factors that influence addiction susceptibility are also known to have a DNA epigenetic basis. In the present review, we discuss the notion that addiction susceptibility has an underlying DNA epigenetic basis. We focus on major phenotypes of addiction susceptibility and review evidence of cell type-specific, time dependent, and sex biased effects of drug use. We highlight the role of DNA epigenetics in these diverse processes and propose its contribution to addiction susceptibility differences. Given the prevalence and lack of effective treatments for addiction, elucidating the DNA epigenetic mechanism of addiction vulnerability may represent an expeditious approach to relieving the addiction disease burden.

Local accumbens in vivo imaging during deep brain stimulation reveals a strategy-dependent amelioration of hedonic feeding

Impulsive overeating is a common, disabling feature of eating disorders. Calcium imaging using fiber photometry has emerged as an in vivo methodology to measure neuronal population activity immune to electrical stimulation artifact from deep brain stimulation (DBS). Thus, when used simultaneously, calcium imaging can elucidate poorly understood DBS mechanisms. We show that nucleus accumbens D1 medial spiny calcium signaling increases in preparation of hedonic feeding of high-fat food. Further, responsive, over continuous, DBS strategies effectively disrupt this activity leading to decreased consumption. Implementation of this methodology to better understand mechanisms of these and other forms of neuromodulation for various indications may help advance the field to identify novel therapeutic targets with applications extending beyond obesity.

Impulsive overeating is a common, disabling feature of eating disorders. Both continuous deep brain stimulation (DBS) and responsive DBS, which limits current delivery to pathological brain states, have emerged as potential therapies. We used in vivo fiber photometry in wild-type, Drd1-cre, and A2a-cre mice to 1) assay subtype-specific medium spiny neuron (MSN) activity of the nucleus accumbens (NAc) during hedonic feeding of high-fat food, and 2) examine DBS strategy-specific effects on NAc activity. D1, but not D2, NAc GCaMP activity increased immediately prior to high-fat food approach. Responsive DBS triggered a GCaMP surge throughout the stimulation period and durably reduced high-fat intake. However, with continuous DBS, this surge decayed, and high-fat intake reemerged. Our results argue for a stimulation strategy-dependent modulation of D1 MSNs with a more sustained decrease in consumption with responsive DBS. This study illustrates the important role in vivo imaging can play in understanding effects of such novel therapies.

Guidelines for the use of flow cytometry and cell sorting in immunological studies (third edition)

The third edition of Flow Cytometry Guidelines provides the key aspects to consider when performing flow cytometry experiments and includes comprehensive sections describing phenotypes and functional assays of all major human and murine immune cell subsets. Notably, the Guidelines contain helpful tables highlighting phenotypes and key differences between human and murine cells. Another useful feature of this edition is the flow cytometry analysis of clinical samples with examples of flow cytometry applications in the context of autoimmune diseases, cancers as well as acute and chronic infectious diseases. Furthermore, there are sections detailing tips, tricks and pitfalls to avoid. All sections are written and peer-reviewed by leading flow cytometry experts and immunologists, making this edition an essential and state-of-the-art handbook for basic and clinical researchers.

From Progenitors to Progeny: Shaping Striatal Circuit Development and Function

Understanding how neurons of the striatum are formed and integrate into complex synaptic circuits is essential to provide insight into striatal function in health and disease. In this review, we summarize our current understanding of the development of striatal neurons and associated circuits with a focus on their embryonic origin. Specifically, we address the role of distinct types of embryonic progenitors, found in the proliferative zones of the ganglionic eminences in the ventral telencephalon, in the generation of diverse striatal interneurons and projection neurons. Indeed, recent evidence would suggest that embryonic progenitor origin dictates key characteristics of postnatal cells, including their neurochemical content, their location within striatum, and their long-range synaptic inputs. We also integrate recent observations regarding embryonic progenitors in cortical and other regions and discuss how this might inform future research on the ganglionic eminences. Last, we examine how embryonic progenitor dysfunction can alter striatal formation, as exemplified in Huntington's disease and autism spectrum disorder, and how increased understanding of embryonic progenitors can have significant implications for future research directions and the development of improved therapeutic options.SIGNIFICANCE STATEMENT This review highlights recently defined novel roles for embryonic progenitor cells in shaping the functional properties of both projection neurons and interneurons of the striatum. It outlines the developmental mechanisms that guide neuronal development from progenitors in the embryonic ganglionic eminences to progeny in the striatum. Where questions remain open, we integrate observations from cortex and other regions to present possible avenues for future research. Last, we provide a progenitor-centric perspective onto both Huntington's disease and autism spectrum disorder. We suggest that future investigations and manipulations of embryonic progenitor cells in both research and clinical settings will likely require careful consideration of their great intrinsic diversity and neurogenic potential.

Methylation of the tyrosine hydroxylase gene is dysregulated by cocaine dependence in the human striatum

Cocaine dependence is a chronic, relapsing disorder caused by lasting changes in the brain. Animal studies have identified cocaine-related alterations in striatal DNA methylation; however, it is unclear how methylation is related to cocaine dependence in humans. We generated methylomic profiles of the nucleus accumbens using human postmortem brains from a cohort of individuals with cocaine dependence and healthy controls (n = 25 per group). We found hypermethylation in a cluster of CpGs within the gene body of tyrosine hydroxylase (TH), containing a putative binding site for the early growth response 1 (EGR1) transcription factor, which is hypermethylated in the caudate nucleus of cocaine-dependent individuals. We replicated this finding and found it to be specific to striatal neuronal nuclei. Furthermore, this locus demonstrates enhancer activity which is attenuated by methylation and enhanced by EGR1 overexpression. These results suggest that cocaine dependence alters the epigenetic regulation of dopaminergic signaling genes.

Astrocyte-neuron interaction in the dorsal striatum-pallidal circuits and alcohol-seeking behaviors

In the striatum, two main types of GABAergic medium spiny neurons (MSNs), denoted striatonigral (or direct-pathway MSNs, dMSNs) and striatopallidal neurons (indirect-pathway MSNs, iMSNs), form circuits with distinct pallidal nuclei, which sends "GO" or "NO-GO" signals through the thalamus. These striatopallidal circuits evaluate and execute reward-seeking and taking behaviors. Especially, the dorsal striatum can be further divided into the dorsomedial striatum (DMS, equivalent to caudate in primates and humans) and dorsolateral striatum (DLS, equivalent to putamen), which orchestrates goal-directed and habitual reward-seeking and taking behaviors, respectively. Using optogenetics, chemogenetics and in vivo calcium imaging technologies combined with electrophysiology and digitalized behavior phenotyping, recent studies have revealed cell-, circuit- and context-specific functions of these microcircuits in addictive behaviors. Also, region-specific astrocytes regulate the homeostatic activities of the dMSNs and iMSNs as well as the downstream circuits, which determine the net balance of cortico-striato-pallidal activities to the thalamic neurons. This review will summarize the recent progress of striatopallidal circuits focusing on astrocyte-neuron interaction and, reward- and alcohol-seeking behaviors. Our review will also discuss the translational and clinical implications of these microcircuit studies. This article is part of the special Issue on "Neurocircuitry Modulating Drug and Alcohol Abuse".

Single-nucleus transcriptome analysis reveals cell-type-specific molecular signatures across reward circuitry in the human brain

Single-cell gene expression technologies are powerful tools to study cell types in the human brain, but efforts have largely focused on cortical brain regions. We therefore created a single-nucleus RNA-sequencing resource of 70,615 high-quality nuclei to generate a molecular taxonomy of cell types across five human brain regions that serve as key nodes of the human brain reward circuitry: nucleus accumbens, amygdala, subgenual anterior cingulate cortex, hippocampus, and dorsolateral prefrontal cortex. We first identified novel subpopulations of interneurons and medium spiny neurons (MSNs) in the nucleus accumbens and further characterized robust GABAergic inhibitory cell populations in the amygdala. Joint analyses across the 107 reported cell classes revealed cell-type substructure and unique patterns of transcriptomic dynamics. We identified discrete subpopulations of D1- and D2-expressing MSNs in the nucleus accumbens to which we mapped cell-type-specific enrichment for genetic risk associated with both psychiatric disease and addiction.

Unbiased identification of novel transcription factors in striatal compartmentation and striosome maturation

Many diseases are linked to dysregulation of the striatum. Striatal function depends on neuronal compartmentation into striosomes and matrix. Striatal projection neurons are GABAergic medium spiny neurons (MSNs), subtyped by selective expression of receptors, neuropeptides, and other gene families. Neurogenesis of the striosome and matrix occurs in separate waves, but the factors regulating compartmentation and neuronal differentiation are largely unidentified. We performed RNA- and ATAC-seq on sorted striosome and matrix cells at postnatal day 3, using the Nr4a1-EGFP striosome reporter mouse. Focusing on the striosome, we validated the localization and/or role of Irx1, Foxf2, Olig2, and Stat1/2 in the developing striosome and the in vivo enhancer function of a striosome-specific open chromatin region 4.4 Kb downstream of Olig2. These data provide novel tools to dissect and manipulate the networks regulating MSN compartmentation and differentiation, including in human iPSC-derived striatal neurons for disease modeling and drug discovery.

Transcriptional downregulation of FAM3C/ILEI in the Alzheimer's brain

Amyloid-β (Aβ) accumulation in the brain triggers the pathogenic cascade for Alzheimer's disease (AD) development. The secretory protein FAM3C (also named ILEI) is a candidate for an endogenous suppressor of Aβ production. In this study, we found that FAM3C expression was transcriptionally downregulated in the AD brain. To determine the transcriptional mechanism of the human FAM3C gene, we delineated the minimal 5'-flanking sequence required for basal promoter activity. From a database search for DNA-binding motifs, expression analysis using cultured cells, and promoter DNA-binding assays, we identified SP1 and EBF1 as candidate basal transcription factors for FAM3C, and found that SMAD1 was a putative inducible transcription factor and KLF6 was a transcription repressor for FAM3C. Genomic deletion of the basal promoter sequence from HEK293 and Neuro-2a cells markedly reduced endogenous expression of FAM3C and abrogated SP1- or EBF1-mediated induction of FAM3C. Nuclear protein extracts from AD brains contained lower levels of SP1 and EBF1 than did those from control brains, although the relative mRNA levels of these factors did not differ significantly between the groups. Additionally, the ability of nuclear SP1 and EBF1 in AD brains to bind with the basal promoter sequence-containing DNA probe was reduced compared with the binding ability of these factors in control brains. Thus, the transcriptional downregulation of FAM3C in the AD brain is attributable to the reduced nuclear levels and genomic DNA binding of SP1 and EBF1. An expressional decline in FAM3C may be a risk factor for Aβ accumulation and eventually AD development.

Localization, proteomics, and metabolite profiling reveal a putative vesicular transporter for UDP-glucose

Vesicular neurotransmitter transporters (VNTs) mediate the selective uptake and enrichment of small-molecule neurotransmitters into synaptic vesicles (SVs) and are therefore a major determinant of the synaptic output of specific neurons. To identify novel VNTs expressed on SVs (thus identifying new neurotransmitters and/or neuromodulators), we conducted localization profiling of 361 solute carrier (SLC) transporters tagging with a fluorescent protein in neurons, which revealed 40 possible candidates through comparison with a known SV marker. We parallelly performed proteomics analysis of immunoisolated SVs and identified seven transporters in overlap. Ultrastructural analysis further supported that one of the transporters, SLC35D3, localized to SVs. Finally, by combining metabolite profiling with a radiolabeled substrate transport assay, we identified UDP-glucose as the principal substrate for SLC35D3. These results provide new insights into the functional role of SLC transporters in neurotransmission and improve our understanding of the molecular diversity of chemical transmitters.

Homeobox Gene Six3 is Required for the Differentiation of D2-Type Medium Spiny Neurons

Medium spiny neurons (MSNs) in the striatum, which can be divided into D1 and D2 MSNs, originate from the lateral ganglionic eminence (LGE). Previously, we reported that Six3 is a downstream target of Sp8/Sp9 in the transcriptional regulatory cascade of D2 MSN development and that conditionally knocking out Six3 leads to a severe loss of D2 MSNs. Here, we showed that Six3 mainly functions in D2 MSN precursor cells and gradually loses its function as D2 MSNs mature. Conditional deletion of Six3 had little effect on cell proliferation but blocked the differentiation of D2 MSN precursor cells. In addition, conditional overexpression of Six3 promoted the differentiation of precursor cells in the LGE. We measured an increase of apoptosis in the postnatal striatum of conditional Six3-knockout mice. This suggests that, in the absence of Six3, abnormally differentiated D2 MSNs are eliminated by programmed cell death. These results further identify Six3 as an important regulatory element during D2 MSN differentiation.

A Cre-Dependent CRISPR/dCas9 System for Gene Expression Regulation in Neurons

Site-specific genetic and epigenetic targeting of distinct cell populations is a central goal in molecular neuroscience and is crucial to understand the gene regulatory mechanisms that underlie complex phenotypes and behaviors. While recent technological advances have enabled unprecedented control over gene expression, many of these approaches are focused on selected model organisms and/or require labor-intensive customization for different applications. The simplicity and modularity of clustered regularly interspaced short palindromic repeats (CRISPR)-based systems have transformed genome editing and expanded the gene regulatory toolbox. However, there are few available tools for cell-selective CRISPR regulation in neurons. We designed, validated, and optimized CRISPR activation (CRISPRa) and CRISPR interference (CRISPRi) systems for Cre recombinase-dependent gene regulation. Unexpectedly, CRISPRa systems based on a traditional double-floxed inverted open reading frame (DIO) strategy exhibited leaky target gene induction even without Cre. Therefore, we developed an intron-containing Cre-dependent CRISPRa system (SVI-DIO-dCas9-VPR) that alleviated leaky gene induction and outperformed the traditional DIO system at endogenous genes in HEK293T cells and rat primary neuron cultures. Using gene-specific CRISPR sgRNAs, we demonstrate that SVI-DIO-dCas9-VPR can activate numerous rat or human genes (GRM2, Tent5b, Fos, Sstr2, and Gadd45b) in a Cre-specific manner. To illustrate the versatility of this tool, we created a parallel CRISPRi construct that successfully inhibited expression from a luciferase reporter in HEK293T cells only in the presence of Cre. These results provide a robust framework for Cre-dependent CRISPR-dCas9 approaches across different model systems, and enable cell-specific targeting when combined with common Cre driver lines or Cre delivery via viral vectors.

Gap Junctions Between Striatal D1 Neurons and Cholinergic Interneurons

The striatum participates in numerous important behaviors. Its principal projection neurons use GABA and peptides as neurotransmitters and interact extensively with interneurons, including cholinergic interneurons (ChIs) that are tonically active. Dissecting the interactions between projection neurons and ChIs is important for uncovering the role and mechanisms of the striatal microcircuits. Here, by combining several optogenetic tools with cell type-specific electrophysiological recordings, we uncovered direct electrical coupling between D1-type projection neurons and ChIs, in addition to the chemical transmission between these two major cell types. Optogenetic stimulation or inhibition led to bilateral current exchanges between D1 neurons and ChIs, which can be abolished by gap junction blockers. We further confirmed the presence of gap junctions through paired electrophysiological recordings and dye microinjections. Finally, we found that activating D1 neurons promotes basal activity of ChIs via gap junctions. Collectively, these results reveal the coexistence of the chemical synapse and gap junctions between D1 neurons and ChIs, which contributes to maintaining the tonically active firing patterns of ChIs.

Individual differences in stereotypy and neuron subtype translatome with TrkB deletion

Motor stereotypies occurring in early-onset neuropsychiatric diseases are associated with dysregulated basal ganglia direct-pathway activity. Disruptions in network connectivity through impaired neuronal structure have been implicated in both rodents and humans. However, the neurobiological mechanisms leading to direct-pathway neuron disconnectivity in stereotypy remain poorly understood. We have a mouse line with Tropomyosin receptor kinase B (TrkB) receptor deletion from D1-expressing cells (D1-Cre-flTrkB) in which a subset of animals shows repetitive rotations and head tics with juvenile onset. Here we demonstrate these behaviors may be associated with abnormal direct-pathway activity by reducing rotations using chemogenetic inhibition of dorsal striatum D1-medium spiny neurons (D1-MSNs) in both juvenile and young-adult mice. Taking advantage of phenotypical differences in animals with similar genotypes, we then interrogated the D1-MSN specific translatome associated with repetitive behavior by using RNA sequencing of ribosome-associated mRNA. Detailed translatome analysis followed by multiplexed gene expression assessment revealed profound alterations in neuronal projection and synaptic structure related genes in stereotypy mice. Examination of neuronal morphology demonstrated dendritic atrophy and dendritic spine loss in dorsal striatum D1-MSNs from mice with repetitive behavior. Together, our results uncover phenotype-specific molecular alterations in D1-MSNs that relate to morphological adaptations in mice displaying stereotypy behavior.

Developmental Characterization of Schizophrenia-Associated Gene Zswim6 in Mouse Forebrain

Schizophrenia is a devastating neuropsychiatric disease with a globally 1% life-long prevalence. Clinical studies have linked Zswim6 mutations to developmental and neurological diseases, including schizophrenia. Zswim6’s function remains largely unknown. Given the involvement of Zswim6 in schizophrenia and schizophrenia as a neurodevelopmental disease, it is important to understand the spatiotemporal expression pattern of Zswim6 in the developing brain. Here, we performed a comprehensive analysis of the spatiotemporal expression pattern of Zswim6 in the mouse forebrain by in situ hybridization with radioactive and non-radioactive-labeled riboprobes. Zswim6 mRNA was detected as early as E11.5 in the ventral forebrain. At E11.5–E13.5, Zswim6 was highly expressed in the lateral ganglionic eminence (LGE). The LGE consisted of two progenitor populations. Dlx⁺;Er81⁺ cells in dorsal LGE comprised progenitors of olfactory bulb interneurons, whereas Dlx⁺;Isl1⁺ progenitors in ventral LGE gave rise to striatal projection neurons. Zswim6 was not colocalized with Er81 in the dorsal LGE. In the ventral LGE, Zswim6 was colocalized with striatal progenitor marker Nolz-1. Zswim6 was highly expressed in the subventricular zone (SVZ) of LGE in which progenitors undergo the transition from proliferation to differentiation. Double labeling showed that Zswim6 was not colocalized with proliferation marker Ki67 but was colocalized with differentiation marker Tuj1 in the SVZ, suggesting Zswim6 expression in early differentiating neurons. Zswim6 was also expressed in the adjacent structures of medial and caudal ganglionic eminences (MGE, CGE) that contained progenitors of cortical interneurons. At E15.5 and E17.5, Zswim6 was expressed in several key brain regions that were involved in the pathogenesis of schizophrenia, including the striatum, cerebral cortex, hippocampus, and medial habenular nucleus. Zswim6 was persistently expressed in the postnatal brain. Cell type analysis indicated that Zswim6 mRNA was colocalized with D1R-expressing striatonigral and D2R-expressing striatopallidal neurons of the adult striatum with a higher colocalization in striatopallidal neurons. These findings are of particular interest as striatal dopamine D2 receptors are known to be involved in the pathophysiology of schizophrenia. In summary, the comprehensive analysis provides an anatomical framework for the study of Zswim6 function and Zswim6-associated neurological disorders.

Striatal Direct Pathway Targets Npas1+ Pallidal Neurons

The classic basal ganglia circuit model asserts a complete segregation of the two striatal output pathways. Empirical data argue that, in addition to indirect-pathway striatal projection neurons (iSPNs), direct-pathway striatal projection neurons (dSPNs) innervate the external globus pallidus (GPe). However, the functions of the latter were not known. In this study, we interrogated the organization principles of striatopallidal projections and their roles in full-body movement in mice (both males and females). In contrast to the canonical motor-promoting response of dSPNs in the dorsomedial striatum (DMSdSPNs), optogenetic stimulation of dSPNs in the dorsolateral striatum (DLSdSPNs) suppressed locomotion. Circuit analyses revealed that dSPNs selectively target Npas1+ neurons in the GPe. In a chronic 6-hydroxydopamine lesion model of Parkinson's disease, the dSPN-Npas1+ projection was dramatically strengthened. As DLSdSPN-Npas1+ projection suppresses movement, the enhancement of this projection represents a circuit mechanism for the hypokinetic symptoms of Parkinson's disease that has not been previously considered. In sum, our results suggest that dSPN input to the GPe is a critical circuit component that is involved in the regulation of movement in both healthy and parkinsonian states.SIGNIFICANCE STATEMENT In the classic basal ganglia model, the striatum is described as a divergent structure: it controls motor and adaptive functions through two segregated, opposing output streams. However, the experimental results that show the projection from direct-pathway neurons to the external pallidum have been largely ignored. Here, we showed that this striatopallidal subpathway targets a select subset of neurons in the external pallidum and is motor-suppressing. We found that this subpathway undergoes changes in a Parkinson's disease model. In particular, our results suggest that the increase in strength of this subpathway contributes to the slowness or reduced movements observed in Parkinson's disease.

Neuroproteomics of the Synapse: Subcellular Quantification of Protein Networks and Signaling Dynamics

One of the most fascinating features of the brain is its ability to adapt to its surroundings. Synaptic plasticity, the dynamic mechanism of functional and structural alterations in synaptic strength, is essential for brain functioning and underlies a variety of processes such as learning and memory. Although the molecular mechanisms underlying such rapid plasticity are not fully understood, a consensus exists on the important role of proteins. The study of these neuronal proteins using neuroproteomics has increased rapidly in the last decades, and advancements in MS-based proteomics have broadened our understanding of neuroplasticity exponentially. In this review, we discuss the trends in MS-based neuroproteomics for the study of synaptic protein-protein interactions and protein signaling dynamics, with a focus on sample types, different labeling and enrichment approaches, and data analysis and interpretation. We highlight studies from the last 5 years, with a focus on synapse structure, composition, functioning, or signaling and finally discuss some recent developments that could further advance the field of neuroproteomics.

Diversity in striatal synaptic circuits arises from distinct embryonic progenitor pools in the ventral telencephalon

Synaptic circuits in the brain are precisely organized, but the processes that govern this precision are poorly understood. Here, we explore how distinct embryonic neural progenitor pools in the lateral ganglionic eminence contribute to neuronal diversity and synaptic circuit connectivity in the mouse striatum. In utero labeling of Tα1-expressing apical intermediate progenitors (aIP), as well as other progenitors (OP), reveals that both progenitors generate direct and indirect pathway spiny projection neurons (SPNs) with similar electrophysiological and anatomical properties and are intermingled in medial striatum. Subsequent optogenetic circuit-mapping experiments demonstrate that progenitor origin significantly impacts long-range excitatory input strength, with medial prefrontal cortex preferentially driving aIP-derived SPNs and visual cortex preferentially driving OP-derived SPNs. In contrast, the strength of local inhibitory inputs among SPNs is controlled by birthdate rather than progenitor origin. Combined, these results demonstrate distinct roles for embryonic progenitor origin in shaping neuronal and circuit properties of the postnatal striatum.

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The Tα1 promoter distinguishes two embryonic progenitor pools in the LGE

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Both pools generate intermixed spiny projection neurons in dorsomedial striatum

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Excitatory cortical inputs are biased toward SPNs of different embryonic origin

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Neurogenic stage rather impacts local inhibitory connections among SPNs

Single-Cell Analysis of Foxp1-Driven Mechanisms Essential for Striatal Development

The striatum is a critical forebrain structure integrating cognitive, sensory, and motor information from diverse brain regions into meaningful behavioral output. However, the transcriptional mechanisms underlying striatal development at single-cell resolution remain unknown. Using single-cell RNA sequencing (RNA-seq), we examine the cellular diversity of the early postnatal striatum and show that Foxp1, a transcription factor strongly linked to autism and intellectual disability, regulates the cellular composition, neurochemical architecture, and connectivity of the striatum in a cell-type-dependent fashion. We also identify Foxp1-regulated target genes within distinct cell types and connect these molecular changes to functional and behavioral deficits relevant to phenotypes described in patients with FOXP1 loss-of-function mutations. Using this approach, we could also examine the non-cell-autonomous effects produced by disrupting one cell type and the molecular compensation that occurs in other populations. These data reveal the cell-type-specific transcriptional mechanisms regulated by Foxp1 that underlie distinct features of striatal circuitry.

How predictive learning influences choice: Evidence for a GPCR-based memory process necessary for Pavlovian-instrumental transfer

Predictive learning endows stimuli with the capacity to signal both the sensory-specific and general motivational properties of their associated rewards or outcomes. These two signals can be distinguished behaviorally by their influence on the selection and performance of instrumental actions, respectively. This review focuses on how sensory-specific predictive learning guides choice between actions that earn otherwise equally desirable outcomes. We describe evidence that outcome-specific predictive learning is encoded in the basolateral amygdala and drives the accumulation of delta-opioid receptors on the surface of cholinergic interneurons located in the nucleus accumbens shell. This accumulation constitutes a novel form of cellular memory, not for outcome-specific predictive learning per se but for the selection of, and choice between, future instrumental actions. We describe recent evidence regarding the cascade of events necessary for the formation and expression of this cellular memory and point to open questions for future research into this process. Beyond these mechanistic considerations, the discovery of this new form of memory is consistent with recent evidence suggesting that intracellular rather than synaptic changes can mediate learning-related plasticity to modify brain circuitry to prepare for future significant events.

Striatal Projection Neurons Require Huntingtin for Synaptic Connectivity and Survival

Huntington's disease (HD) is caused by an autosomal dominant polyglutamine expansion mutation of Huntingtin (HTT). HD patients suffer from progressive motor, cognitive, and psychiatric impairments, along with significant degeneration of the striatal projection neurons (SPNs) of the striatum. HD is widely accepted to be caused by a toxic gain-of-function of mutant HTT. However, whether loss of HTT function, because of dominant-negative effects of the mutant protein, plays a role in HD and whether HTT is required for SPN health and function are not known. Here, we delete Htt from specific subpopulations of SPNs using the Cre-Lox system and find that SPNs require HTT for motor regulation, synaptic development, cell health, and survival during aging. Our results suggest that loss of HTT function in SPNs could play a critical role in HD pathogenesis.

Drug-activated cells: From immediate early genes to neuronal ensembles in addiction

Beyond their rapid rewarding effects, drugs of abuse can durably alter an individual's response to their environment as illustrated by the compulsive drug seeking and risk of relapse triggered by drug-associated stimuli. The persistence of these associations even long after cessation of drug use demonstrates the enduring mark left by drugs on brain reward circuits. However, within these circuits, neuronal populations are differently affected by drug exposure and growing evidence indicates that relatively small subsets of neurons might be involved in the encoding and expression of drug-mediated associations. The identification of sparse neuronal populations recruited in response to drug exposure has benefited greatly from the study of immediate early genes (IEGs) whose induction is critical in initiating plasticity programs in recently activated neurons. In particular, the development of technologies to manipulate IEG-expressing cells has been fundamental to implicate broadly distributed neuronal ensembles coincidently activated by either drugs or drug-associated stimuli and to then causally establish their involvement in drug responses. In this review, we summarize the literature regarding IEG regulation in different learning paradigms and addiction models to highlight their role as a marker of activity and plasticity. As the exploration of neuronal ensembles in addiction improves our understanding of drug-associated memory encoding, it also raises several questions regarding the cellular and molecular characteristics of these discrete neuronal populations as they become incorporated in drug-associated neuronal ensembles. We review recent efforts towards this goal and discuss how they will offer a more comprehensive understanding of addiction pathophysiology.

Generation of Human Neurons by microRNA-Mediated Direct Conversion of Dermal Fibroblasts

MicroRNAs (miRNAs), miR-9/9^*, and miR-124 (miR-9/9^*-124) display fate-reprogramming activities when ectopically expressed in human fibroblasts by erasing the fibroblast identity and evoking a pan-neuronal state. In contrast to induced pluripotent stem cell-derived neurons, miRNA-induced neurons (miNs) retain the biological age of the starting fibroblasts through direct fate conversion and thus provide a human neuron-based platform to study cellular properties inherent in aged neurons and model adult-onset neurodegenerative disorders using patient-derived cells. Furthermore, expression of neuronal subtype-specific transcription factors in conjunction with miR-9/9^*-124 guides the miNs to distinct neuronal fates, a feature critical for modeling disorders that affect specific neuronal subtypes. Here, we describe the miR-9/9^*-124-based neuronal reprogramming protocols for the generation of several disease-relevant neuronal subtypes: striatal medium spiny neurons, cortical neurons, and spinal cord motor neurons.

Shisa6 mediates cell-type specific regulation of depression in the nucleus accumbens

Depression is the leading cause of disability and produces enormous health and economic burdens. Current treatment approaches for depression are largely ineffective and leave more than 50% of patients symptomatic, mainly because of non-selective and broad action of antidepressants. Thus, there is an urgent need to design and develop novel therapeutics to treat depression. Given the heterogeneity and complexity of the brain, identification of molecular mechanisms within specific cell-types responsible for producing depression-like behaviors will advance development of therapies. In the reward circuitry, the nucleus accumbens (NAc) is a key brain region of depression pathophysiology, possibly based on differential activity of D1- or D2- medium spiny neurons (MSNs). Here we report a circuit- and cell-type specific molecular target for depression, Shisa6, recently defined as an AMPAR component, which is increased only in D1-MSNs in the NAc of susceptible mice. Using the Ribotag approach, we dissected the transcriptional profile of D1- and D2-MSNs by RNA sequencing following a mouse model of depression, chronic social defeat stress (CSDS). Bioinformatic analyses identified cell-type specific genes that may contribute to the pathogenesis of depression, including Shisa6. We found selective optogenetic activation of the ventral tegmental area (VTA) to NAc circuit increases Shisa6 expression in D1-MSNs. Shisa6 is specifically located in excitatory synapses of D1-MSNs and increases excitability of neurons, which promotes anxiety- and depression-like behaviors in mice. Cell-type and circuit-specific action of Shisa6, which directly modulates excitatory synapses that convey aversive information, identifies the protein as a potential rapid-antidepressant target for aberrant circuit function in depression.

Neural Mechanisms Underlying the Rewarding and Therapeutic Effects of Ketamine as a Treatment for Alcohol Use Disorder

Alcohol use disorder (AUD) is the most prevalent substance use disorder and causes a significant global burden. Relapse rates remain incredibly high after decades of attempting to develop novel treatment options that have failed to produce increased rates of sobriety. Ketamine has emerged as a potential treatment for AUD following its success as a therapeutic agent for depression, demonstrated by several preclinical studies showing that acute administration reduced alcohol intake in rodents. As such, ketamine's therapeutic effects for AUD are now being investigated in clinical trials with the hope of it being efficacious in prolonging sobriety from alcohol in humans (ClinicalTrials.gov, Identifier: NCT01558063). Importantly, ketamine's antidepressant effects only last for about 1-week and because AUD is a lifelong disorder, repeated treatment regimens would be necessary to maintain sobriety. This raises questions regarding its safety for AUD treatment since ketamine itself has the potential for addiction. Therefore, this review aims to summarize the neuroadaptations related to alcohol's addictive properties as well as ketamine's therapeutic and addictive properties. To do this, the focus will be on reward-related brain regions such as the nucleus accumbens (NAc), dorsal striatum, prefrontal cortex (PFC), hippocampus, and ventral tegmental area (VTA) to understand how acute vs. chronic exposure will alter reward signaling over time. Additionally, evidence from these studies will be summarized in both male and female subjects. Accordingly, this review aims to address the safety of repeated ketamine infusions for the treatment of AUD. Although more work about the safety of ketamine to treat AUD is warranted, we hope this review sheds light on some answers about the safety of repeated ketamine infusions.

A Molecular Calcium Integrator Reveals a Striatal Cell Type Driving Aversion

The ability to record transient cellular events in the DNA or RNA of cells would enable precise, large-scale analysis, selection, and reprogramming of heterogeneous cell populations. Here, we report a molecular technology for stable genetic tagging of cells that exhibit activity-related increases in intracellular calcium concentration (FLiCRE). We used FLiCRE to transcriptionally label activated neural ensembles in the nucleus accumbens of the mouse brain during brief stimulation of aversive inputs. Using single-cell RNA sequencing, we detected FLiCRE transcripts among the endogenous transcriptome, providing simultaneous readout of both cell-type and calcium activation history. We identified a cell type in the nucleus accumbens activated downstream of long-range excitatory projections. Taking advantage of FLiCRE's modular design, we expressed an optogenetic channel selectively in this cell type and showed that direct recruitment of this otherwise genetically inaccessible population elicits behavioral aversion. The specificity and minute resolution of FLiCRE enables molecularly informed characterization, manipulation, and reprogramming of activated cellular ensembles.