Thalamocortical organoids enable in vitro modeling of 22q11.2 microdeletion associated with neuropsychiatric disorders.

Thalamic dysfunction has been implicated in multiple psychiatric disorders. We sought to study the mechanisms by which abnormalities emerge in the context of the 22q11.2 microdeletion, which confers significant genetic risk for psychiatric disorders. We investigated early stages of human thalamus development using human pluripotent stem cell-derived organoids and show that the 22q11.2 microdeletion underlies widespread transcriptional dysregulation associated with psychiatric disorders in thalamic neurons and glia, including elevated expression of FOXP2. Using an organoid co-culture model, we demonstrate that the 22q11.2 microdeletion mediates an overgrowth of thalamic axons in a FOXP2-dependent manner. Finally, we identify ROBO2 as a candidate molecular mediator of the effects of FOXP2 overexpression on thalamic axon overgrowth. Together, our study suggests that early steps in thalamic development are dysregulated in a model of genetic risk for schizophrenia and contribute to neural phenotypes in 22q11.2 deletion syndrome.

Amygdala-hippocampus somatostatin interneuron beta-synchrony underlies a cross-species biomarker of emotional state.

Emotional responses arise from limbic circuits including the hippocampus and amygdala. In the human brain, beta-frequency communication between these structures correlates with self-reported mood and anxiety. However, both the mechanism and significance of this biomarker as a readout vs. driver of emotional state remain unknown. Here, we show that beta-frequency communication between ventral hippocampus and basolateral amygdala also predicts anxiety-related behavior in mice, both on long timescales (∼30 min) and immediately preceding behavioral choices. Genetically encoded voltage indicators reveal that this biomarker reflects synchronization between somatostatin interneurons across both structures. Indeed, synchrony between these neurons dynamically predicts approach-avoidance decisions, and optogenetically shifting the phase of synchronization by just 25 ms is sufficient to bidirectionally modulate anxiety-related behaviors. Thus, back-translation establishes a human biomarker as a causal determinant (not just predictor) of emotional state, revealing a novel mechanism whereby interregional synchronization that is frequency, phase, and cell type specific controls emotional processing.

Long-range inhibition synchronizes and updates prefrontal task activity.

Changes in patterns of activity within the medial prefrontal cortex enable rodents, non-human primates and humans to update their behaviour to adapt to changes in the environment-for example, during cognitive tasks1-5. Parvalbumin-expressing inhibitory neurons in the medial prefrontal cortex are important for learning new strategies during a rule-shift task6-8, but the circuit interactions that switch prefrontal network dynamics from maintaining to updating task-related patterns of activity remain unknown. Here we describe a mechanism that links parvalbumin-expressing neurons, a new callosal inhibitory connection, and changes in task representations. Whereas nonspecifically inhibiting all callosal projections does not prevent mice from learning rule shifts or disrupt the evolution of activity patterns, selectively inhibiting only callosal projections of parvalbumin-expressing neurons impairs rule-shift learning, desynchronizes the gamma-frequency activity that is necessary for learning8 and suppresses the reorganization of prefrontal activity patterns that normally accompanies rule-shift learning. This dissociation reveals how callosal parvalbumin-expressing projections switch the operating mode of prefrontal circuits from maintenance to updating by transmitting gamma synchrony and gating the ability of other callosal inputs to maintain previously established neural representations. Thus, callosal projections originating from parvalbumin-expressing neurons represent a key circuit locus for understanding and correcting the deficits in behavioural flexibility and gamma synchrony that have been implicated in schizophrenia and related conditions9,10.

Tsc1 Loss in VIP-Lineage Cortical Interneurons Results in More VIP+ Interneurons and Enhanced Excitability.

The mammalian target of rapamycin (mTOR) signaling pathway is a powerful regulator of cell proliferation, growth, synapse maintenance and cell fate. While intensely studied for its role in cancer, the role of mTOR signaling is just beginning to be uncovered in specific cell types that are implicated in neurodevelopmental disorders. Previously, loss of the Tsc1 gene, which results in hyperactive mTOR, was shown to affect the function and molecular properties of GABAergic cortical interneurons (CINs) derived from the medial ganglionic eminence. To assess if other important classes of CINs could be impacted by mTOR dysfunction, we deleted Tsc1 in a caudal ganglionic eminence-derived interneuron group, the vasoactive intestinal peptide (VIP)+ subtype, whose activity disinhibits local circuits. Tsc1 mutant VIP+ CINs reduced their pattern of apoptosis from postnatal days 15-20, resulting in increased VIP+ CINs. The mutant CINs exhibited synaptic and electrophysiological properties that could contribute to the high rate of seizure activity in humans that harbor Tsc1 mutations.

Selective Inhibitory Circuit Dysfunction after Chronic Frontal Lobe Contusion.

Traumatic brain injury (TBI) is a leading cause of neurologic disability; the most common deficits affect prefrontal cortex-dependent functions such as attention, working memory, social behavior, and mental flexibility. Despite this prevalence, little is known about the pathophysiology that develops in frontal cortical microcircuits after TBI. We investigated whether alterations in subtype-specific inhibitory circuits are associated with cognitive inflexibility in a mouse model of frontal lobe contusion in both male and female mice that recapitulates aberrant mental flexibility as measured by deficits in rule reversal learning. Using patch-clamp recordings and optogenetic stimulation, we identified selective vulnerability in the non-fast-spiking and somatostatin-expressing (SOM+) subtypes of inhibitory neurons in layer V of the orbitofrontal cortex 2 months after injury. These subtypes exhibited reduced intrinsic excitability and a decrease in their synaptic output onto pyramidal neurons, respectively. By contrast, the fast-spiking and parvalbumin-expressing interneurons did not show changes in intrinsic excitability or synaptic output, respectively. Impairments in non-fast-spiking/SOM+ inhibitory circuit function were also associated with network hyperexcitability. These findings provide evidence for selective disruptions within specific inhibitory microcircuits that may guide the development of novel therapeutics for TBI.SIGNIFICANCE STATEMENT TBI frequently leads to chronic deficits in cognitive and behavioral functions that involve the prefrontal cortex, yet the maladaptive changes that occur in these cortical microcircuits are unknown. Our data indicate that alterations in subtype-specific inhibitory circuits, specifically vulnerability in the non-fast-spiking/somatostatin-expressing interneurons, occurs in the orbitofrontal cortex in the context of chronic deficits in reversal learning. These neurons exhibit reduced excitability and synaptic output, whereas the other prominent inhibitory population in layer V, the fast-spiking/parvalbumin-expressing interneurons as well as pyramidal neurons are not affected. Our work offers mechanistic insight into the subtype-specific function of neurons that may contribute to mental inflexibility after TBI.

Fate mapping of neural stem cell niches reveals distinct origins of human cortical astrocytes.

Progenitors of the developing human neocortex reside in the ventricular and outer subventricular zones (VZ and OSVZ, respectively). However, whether cells derived from these niches have similar developmental fates is unknown. By performing fate mapping in primary human tissue, we demonstrate that astrocytes derived from these niches populate anatomically distinct layers. Cortical plate astrocytes emerge from VZ progenitors and proliferate locally, while putative white matter astrocytes are morphologically heterogeneous and emerge from both VZ and OSVZ progenitors. Furthermore, via single-cell sequencing of morphologically defined astrocyte subtypes using Patch-seq, we identify molecular distinctions between VZ-derived cortical plate astrocytes and OSVZ-derived white matter astrocytes that persist into adulthood. Together, our study highlights a complex role for cell lineage in the diversification of human neocortical astrocytes.

Transforming Discoveries About Cortical Microcircuits and Gamma Oscillations Into New Treatments for Cognitive Deficits in Schizophrenia.

The major cause of disability in schizophrenia is cognitive impairment, which remains largely refractory to existing treatments. This reflects the fact that antipsychotics and other therapies have not been designed to address specific brain abnormalities that cause cognitive impairment. This overview proposes that understanding how specific cellular and synaptic loci within cortical microcircuits contribute to cortical gamma oscillations may reveal treatments for cognitive impairment. Gamma oscillations are rhythmic patterns of high frequency (∼30-100 Hz) neuronal activity that are synchronized within and across brain regions, generated by a class of inhibitory interneurons that express parvalbumin, and recruited during a variety of cognitive tasks. In schizophrenia, both parvalbumin interneuron function and task-evoked gamma oscillations are deficient. While it has long been controversial whether gamma oscillations are merely a biomarker of circuit function or actually contribute to information processing by neuronal networks, recent neurobiological studies in mice have shown that disrupting or enhancing synchronized gamma oscillations can reproduce or ameliorate cognitive deficits resembling those seen in schizophrenia. In fact, transiently enhancing the synchrony of parvalbumin interneuron-generated gamma oscillations can lead to long-lasting improvements in cognition in mice that model aspects of schizophrenia. Gamma oscillations emerge from specific patterns of connections between a variety of cell types within cortical microcircuits. Thus, a critical next step is to understand how specific cell types and synapses generate gamma oscillations, mediate the effects of gamma oscillations on information processing, and/or undergo plasticity following the induction of gamma oscillations. Modulating these circuit loci, potentially in combination with other approaches such as cognitive training and brain stimulation, may yield potent and selective interventions for enhancing cognition in schizophrenia.

Top-down control of hippocampal signal-to-noise by prefrontal long-range inhibition.

Prefrontal cortex (PFC) is postulated to exert "top-down control" on information processing throughout the brain to promote specific behaviors. However, pathways mediating top-down control remain poorly understood. In particular, knowledge about direct prefrontal connections that might facilitate top-down control of hippocampal information processing remains sparse. Here we describe monosynaptic long-range GABAergic projections from PFC to hippocampus. These preferentially inhibit vasoactive intestinal polypeptide-expressing interneurons, which are known to disinhibit hippocampal microcircuits. Indeed, stimulating prefrontal-hippocampal GABAergic projections increases hippocampal feedforward inhibition and reduces hippocampal activity in vivo. The net effect of these actions is to specifically enhance the signal-to-noise ratio for hippocampal encoding of object locations and augment object-induced increases in spatial information. Correspondingly, activating or inhibiting these projections promotes or suppresses object exploration, respectively. Together, these results elucidate a top-down prefrontal pathway in which long-range GABAergic projections target disinhibitory microcircuits, thereby enhancing signals and network dynamics underlying exploratory behavior.

Convergence of Clinically Relevant Manipulations on Dopamine-Regulated Prefrontal Activity Underlying Stress Coping Responses.

BACKGROUND: Depression is pleiotropic and influenced by diverse genetic, environmental, and pharmacological factors. Identifying patterns of circuit activity on which many of these factors converge would be important, because studying these patterns could reveal underlying pathophysiological processes and/or novel therapies. Depression is commonly assumed to involve changes within prefrontal circuits, and dopamine D2 receptor (D2R) agonists are increasingly used as adjunctive antidepressants. Nevertheless, how D2Rs influence disease-relevant patterns of prefrontal circuit activity remains unknown. METHODS: We used brain slice calcium imaging to measure how patterns of prefrontal activity are modulated by D2Rs, antidepressants, and manipulations that increase depression susceptibility. To validate the idea that prefrontal D2Rs might contribute to antidepressant responses, we used optogenetic and genetic manipulations to test how dopamine, D2Rs, and D2R+ neurons contribute to stress-coping behavior. RESULTS: Patterns of positively correlated activity in prefrontal microcircuits are specifically enhanced by D2R stimulation as well as by two mechanistically distinct antidepressants, ketamine and fluoxetine. Conversely, this D2R-driven effect was disrupted in two etiologically distinct depression models, a genetic susceptibility model and mice that are susceptible to chronic social defeat. Phasic stimulation of dopaminergic afferents to the prefrontal cortex and closed-loop stimulation of D2R+ neurons increased effortful responses to tail suspension stress, whereas prefrontal D2R deletion reduced the duration of individual struggling episodes. CONCLUSIONS: Correlated prefrontal microcircuit activity represents a point of convergence for multiple depression-related manipulations. Prefrontal D2Rs enhance this activity. Through this mechanism, prefrontal D2Rs may promote network states associated with antidepressant actions and effortful responses to stress.

Dynamic patterns of correlated activity in the prefrontal cortex encode information about social behavior.

New technologies make it possible to measure activity from many neurons simultaneously. One approach is to analyze simultaneously recorded neurons individually, then group together neurons which increase their activity during similar behaviors into an "ensemble." However, this notion of an ensemble ignores the ability of neurons to act collectively and encode and transmit information in ways that are not reflected by their individual activity levels. We used microendoscopic GCaMP imaging to measure prefrontal activity while mice were either alone or engaged in social interaction. We developed an approach that combines a neural network classifier and surrogate (shuffled) datasets to characterize how neurons synergistically transmit information about social behavior. Notably, unlike optimal linear classifiers, a neural network classifier with a single linear hidden layer can discriminate network states which differ solely in patterns of coactivity, and not in the activity levels of individual neurons. Using this approach, we found that surrogate datasets which preserve behaviorally specific patterns of coactivity (correlations) outperform those which preserve behaviorally driven changes in activity levels but not correlated activity. Thus, social behavior elicits increases in correlated activity that are not explained simply by the activity levels of the underlying neurons, and prefrontal neurons act collectively to transmit information about socialization via these correlations. Notably, this ability of correlated activity to enhance the information transmitted by neuronal ensembles is diminished in mice lacking the autism-associated gene Shank3. These results show that synergy is an important concept for the coding of social behavior which can be disrupted in disease states, reveal a specific mechanism underlying this synergy (social behavior increases correlated activity within specific ensembles), and outline methods for studying how neurons within an ensemble can work together to encode information.

Information diversity in individual auditory cortical neurons is associated with functionally distinct coordinated neuronal ensembles.

Neuronal activity in auditory cortex is often highly synchronous between neighboring neurons. Such coordinated activity is thought to be crucial for information processing. We determined the functional properties of coordinated neuronal ensembles (cNEs) within primary auditory cortical (AI) columns relative to the contributing neurons. Nearly half of AI cNEs showed robust spectro-temporal receptive fields whereas the remaining cNEs showed little or no acoustic feature selectivity. cNEs can therefore capture either specific, time-locked information of spectro-temporal stimulus features or reflect stimulus-unspecific, less-time specific processing aspects. By contrast, we show that individual neurons can represent both of those aspects through membership in multiple cNEs with either high or absent feature selectivity. These associations produce functionally heterogeneous spikes identifiable by instantaneous association with different cNEs. This demonstrates that single neuron spike trains can sequentially convey multiple aspects that contribute to cortical processing, including stimulus-specific and unspecific information.

Regulatory Elements Inserted into AAVs Confer Preferential Activity in Cortical Interneurons.

Cortical interneuron (CIN) dysfunction is thought to play a major role in neuropsychiatric conditions like epilepsy, schizophrenia and autism. It is therefore essential to understand how the development, physiology, and functions of CINs influence cortical circuit activity and behavior in model organisms such as mice and primates. While transgenic driver lines are powerful tools for studying CINs in mice, this technology is limited in other species. An alternative approach is to use viral vectors such as AAV, which can be used in multiple species including primates and also have potential for therapeutic use in humans. Thus, we sought to discover gene regulatory enhancer elements (REs) that can be used in viral vectors to drive expression in specific cell types. The present study describes the systematic genome-wide identification of putative REs (pREs) that are preferentially active in immature CINs by histone modification chromatin immunoprecipitation and sequencing (ChIP-seq). We evaluated two novel pREs in AAV vectors, alongside the well-established Dlx I12b enhancer, and found that they drove CIN-specific reporter expression in adult mice. We also showed that the identified Arl4d pRE could drive sufficient expression of channelrhodopsin for optogenetic rescue of behavioral deficits in the Dlx5/6+/- mouse model of fast-spiking CIN dysfunction.

GluN2D-mediated excitatory drive onto medial prefrontal cortical PV+ fast-spiking inhibitory interneurons.

Deficits in fast-spiking inhibitory interneurons (FSINs) within the dorsolateral prefrontal cortex (dlPFC) are hypothesized to underlie cognitive impairment associated with schizophrenia. Though representing a minority of interneurons, this key cell type coordinates broad neural network gamma-frequency oscillations, associated with cognition and cognitive flexibility. Here we report expression of GluN2D mRNA selectively in parvalbumin positive cells of human postmortem dlPFC tissue, but not pyramidal neurons, with little to no GluN2C expression in either cell type. In acute murine mPFC slices the GluN2C/D selective positive allosteric modulator (PAM), CIQ(+), increased the intrinsic excitability as well as enhanced NMDAR-mediated EPSCs onto FSINs. This increase in intrinsic excitability with GluN2C/D PAM was also observed in the Dlx 5/6+/- FSIN developmental deficit model with reported FSIN hypoexcitability. Together these data speak to selective modulation of FSINs by a GluN2D PAM, providing a potential mechanism to counter the FSIN-deficit seen in schizophrenia.

Cross-hemispheric gamma synchrony between prefrontal parvalbumin interneurons supports behavioral adaptation during rule shift learning.

Organisms must learn new strategies to adapt to changing environments. Activity in different neurons often exhibits synchronization that can dynamically enhance their communication and might create flexible brain states that facilitate changes in behavior. We studied the role of gamma-frequency (~40 Hz) synchrony between prefrontal parvalbumin (PV) interneurons in mice learning multiple new cue-reward associations. Voltage indicators revealed cell-type-specific increases of cross-hemispheric gamma synchrony between PV interneurons when mice received feedback that previously learned associations were no longer valid. Disrupting this synchronization by delivering out-of-phase optogenetic stimulation caused mice to perseverate on outdated associations, an effect not reproduced by in-phase stimulation or out-of-phase stimulation at other frequencies. Gamma synchrony was specifically required when new associations used familiar cues that were previously irrelevant to behavioral outcomes, not when associations involved new cues or for reversing previously learned associations. Thus, gamma synchrony is indispensable for reappraising the behavioral salience of external cues.

Enhancing WNT Signaling Restores Cortical Neuronal Spine Maturation and Synaptogenesis in Tbr1 Mutants.

Tbr1 is a high-confidence autism spectrum disorder (ASD) gene encoding a transcription factor with distinct pre- and postnatal functions. Postnatally, Tbr1 conditional knockout (CKO) mutants and constitutive heterozygotes have immature dendritic spines and reduced synaptic density. Tbr1 regulates expression of several genes that underlie synaptic defects, including a kinesin (Kif1a) and a WNT-signaling ligand (Wnt7b). Furthermore, Tbr1 mutant corticothalamic neurons have reduced thalamic axonal arborization. LiCl and a GSK3ß inhibitor, two WNT-signaling agonists, robustly rescue the dendritic spines and the synaptic and axonal defects, suggesting that this could have relevance for therapeutic approaches in some forms of ASD.

Interneuron Transplantation Rescues Social Behavior Deficits without Restoring Wild-Type Physiology in a Mouse Model of Autism with Excessive Synaptic Inhibition.

Manipulations that enhance GABAergic inhibition have been associated with improved behavioral phenotypes in autism models, suggesting that autism may be treated by correcting underlying deficits of inhibition. Interneuron transplantation is a method for increasing recipient synaptic inhibition, and it has been considered a prospective therapy for conditions marked by deficient inhibition, including neuropsychiatric disorders. It is unknown, however, whether interneuron transplantation may be therapeutically effective only for conditions marked by reduced inhibition, and it is also unclear whether transplantation improves behavioral phenotypes solely by normalizing underlying circuit defects. To address these questions, we studied the effects of interneuron transplantation in male and female mice lacking the autism-associated gene, Pten, in GABAergic interneurons. Pten mutant mice exhibit social behavior deficits, elevated synaptic inhibition in prefrontal cortex, abnormal baseline and social interaction-evoked electroencephalogram (EEG) signals, and an altered composition of cortical interneuron subtypes. Transplantation of wild-type embryonic interneurons from the medial ganglionic eminence into the prefrontal cortex of neonatal Pten mutants rescued social behavior despite exacerbating excessive levels of synaptic inhibition. Furthermore, transplantation did not normalize recipient EEG signals measured during baseline states. Interneuron transplantation can thus correct behavioral deficits even when those deficits are associated with elevated synaptic inhibition. Moreover, transplantation does not exert therapeutic effects solely by restoring wild-type circuit states. Our findings indicate that interneuron transplantation could offer a novel cell-based approach to autism treatment while challenging assumptions that effective therapies must reverse underlying circuit defects.SIGNIFICANCE STATEMENT Imbalances between neural excitation and inhibition are hypothesized to contribute to the pathophysiology of autism. Interneuron transplantation is a method for altering recipient inhibition, and it has been considered a prospective therapy for neuropsychiatric disorders, including autism. Here we examined the behavioral and physiological effects of interneuron transplantation in a mouse genetic model of autism. They demonstrate that transplantation rescues recipient social interaction deficits without correcting a common measure of recipient inhibition, or circuit-level physiological measures. These findings demonstrate that interneuron transplantation can exert therapeutic behavioral effects without necessarily restoring wild-type circuit states, while highlighting the potential of interneuron transplantation as an autism therapy.

Integrated Stress Response Inhibitor Reverses Sex-Dependent Behavioral and Cell-Specific Deficits after Mild Repetitive Head Trauma.

Mild repetitive traumatic brain injury (rTBI) induces chronic behavioral and cognitive alterations and increases the risk for dementia. Currently, there are no therapeutic strategies to prevent or mitigate chronic deficits associated with rTBI. Previously we developed an animal model of rTBI that recapitulates the cognitive and behavioral deficits observed in humans. We now report that rTBI results in an increase in risk-taking behavior in male but not female mice. This behavioral phenotype is associated with chronic activation of the integrated stress response and cell-specific synaptic alterations in the type A subtype of layer V pyramidal neurons in the medial prefrontal cortex. Strikingly, by briefly treating animals weeks after injury with ISRIB, a selective inhibitor of the integrated stress response (ISR), we (1) relieve ISR activation, (2) reverse the increased risk-taking behavioral phenotype and maintain this reversal, and (3) restore cell-specific synaptic function in the affected mice. Our results indicate that targeting the ISR even at late time points after injury can permanently reverse behavioral changes. As such, pharmacological inhibition of the ISR emerges as a promising avenue to combat rTBI-induced behavioral dysfunction.

Tsc1 represses parvalbumin expression and fast-spiking properties in somatostatin lineage cortical interneurons.

Medial ganglionic eminence (MGE)-derived somatostatin (SST)+ and parvalbumin (PV)+ cortical interneurons (CINs), have characteristic molecular, anatomical and physiological properties. However, mechanisms regulating their diversity remain poorly understood. Here, we show that conditional loss of the Tuberous Sclerosis Complex (TSC) gene, Tsc1, which inhibits the mammalian target of rapamycin (MTOR), causes a subset of SST+ CINs, to express PV and adopt fast-spiking (FS) properties, characteristic of PV+ CINs. Milder intermediate phenotypes also occur when only one allele of Tsc1 is deleted. Notably, treatment of adult mice with rapamycin, which inhibits MTOR, reverses the phenotypes. These data reveal novel functions of MTOR signaling in regulating PV expression and FS properties, which may contribute to TSC neuropsychiatric symptoms. Moreover, they suggest that CINs can exhibit properties intermediate between those classically associated with PV+ or SST+ CINs, which may be dynamically regulated by the MTOR signaling.

Excitation-inhibition balance as a framework for investigating mechanisms in neuropsychiatric disorders.

In 2003 Rubenstein and Merzenich hypothesized that some forms of Autism (ASD) might be caused by a reduction in signal-to-noise in key neural circuits, which could be the result of changes in excitatory-inhibitory (E-I) balance. Here, we have clarified the concept of E-I balance, and updated the original hypothesis in light of the field's increasingly sophisticated understanding of neuronal circuits. We discuss how specific developmental mechanisms, which reduce inhibition, affect cortical and hippocampal functions. After describing how mutations of some ASD genes disrupt inhibition in mice, we close by suggesting that E-I balance represents an organizing framework for understanding findings related to pathophysiology and for identifying appropriate treatments.