Genetic targeting of principal neurons in neocortex and hippocampus of NEX-Cre mice

Regulation of PV interneuron plasticity by neuropeptide-encoding genes

Neuronal activity must be regulated in a narrow permissive band for the proper operation of neural networks. Changes in synaptic connectivity and network activity-for example, during learning-might disturb this balance, eliciting compensatory mechanisms to maintain network function1-3. In the neocortex, excitatory pyramidal cells and inhibitory interneurons exhibit robust forms of stabilizing plasticity. However, although neuronal plasticity has been thoroughly studied in pyramidal cells4-8, little is known about how interneurons adapt to persistent changes in their activity. Here we describe a critical cellular process through which cortical parvalbumin-expressing (PV+) interneurons adapt to changes in their activity levels. We found that changes in the activity of individual PV+ interneurons drive bidirectional compensatory adjustments of the number and strength of inhibitory synapses received by these cells, specifically from other PV+ interneurons. High-throughput profiling of ribosome-associated mRNA revealed that increasing the activity of a PV+ interneuron leads to upregulation of two genes encoding multiple secreted neuropeptides: Vgf and Scg2. Functional experiments demonstrated that VGF is critically required for the activity-dependent scaling of inhibitory PV+ synapses onto PV+ interneurons. Our findings reveal an instructive role for neuropeptide-encoding genes in regulating synaptic connections among PV+ interneurons in the adult mouse neocortex.

Primary ciliary protein kinase A activity in the prefrontal cortex modulates stress in mice

Primary cilia are cellular antennae emanating from vertebrate cell surfaces to sense and transduce extracellular signals intracellularly to regulate cell behavior and function. However, their signal sensing and physiological functions in neocortical neurons remain largely unclear. Here, we show that, in response to various animal stressors, primary cilia in the mouse prefrontal cortex (PFC) exhibit consistent axonemal elongation. Selective removal of excitatory neuron primary cilia in the prefrontal but not sensory cortex leads to a reduction in animal stress sensing and response. Treatment with corticosterone, the major stress hormone, elicits an increase in primary ciliary cyclic adenosine 3',5'-monphosphate (cAMP) level in PFC excitatory neurons and a decrease in neuronal excitability dependent on primary cilia. Suppression of primary ciliary protein kinase A (PKA) activity in PFC excitatory neurons reduces animal stress. These results suggest that excitatory neurons in the PFC are involved in sensing and regulating animal stress via primary ciliary cAMP/PKA signaling.

HYTANE-Identified Latrophilin-3 Cleavage by Meprin β Leads to Loss of the Interaction Domains

The metalloprotease meprin β is upregulated in neurons and astrocytes of Alzheimer's disease patients' brains. While the role of meprin β as the β-secretase of amyloid precursor protein (APP) has been characterized, its broader substrate profile within the brain remains largely unexplored. Hence, to identify additional substrates, we conducted N-terminomics of brain lysates from mice overexpressing meprin β in astrocytes employing the Hydrophobic Tagging-Assisted N-terminal Enrichment (HYTANE) strategy. We observed 3906 (82.2%) N-terminal peptides and identified seven new substrates that match meprin β in terms of localization and cleavage specificity. Of note, the meprin β overexpressing mice show mild cognitive impairments caused by amyloidogenic APP processing alongside hyperactivity and altered exploratory behavior seemingly independent of APP cleavage. Hence, latrophilin-3 was of particular interest, as latrophilin-3 defects are associated with hyperactivity in mice and human. In brain lysates from mice overexpressing meprin β in astrocytes as well as in cellulo, we validated the cleavage of latrophilin-3, resulting in the release of two N-terminal domains. These domains promote interactions with neuronal proteins such as fibronectin leucine-rich repeat transmembrane proteins, promoting adequate synapse formation. Thus, meprin β might affect synaptic integrity by cleaving interaction domains of latrophilin-3, potentially exacerbating the observed hyperactivity phenotype.

From initial formation to developmental refinement: GABAergic inputs shape neuronal subnetworks in the primary somatosensory cortex

Neuronal subnetworks, also known as ensembles, are functional units formed by interconnected neurons for information processing and encoding in the adult brain. Our study investigates the establishment of neuronal subnetworks in the mouse primary somatosensory (S1) cortex from postnatal days (P)11 to P21 using in vivo two-photon calcium imaging. We found that at P11, neuronal activity was highly synchronized but became sparser by P21. Clustering analyses revealed that while the number of subnetworks remained constant, their activity patterns became more distinct, with increased coherence, independent of cortical layer or sex. Furthermore, the coherence of neuronal activity within individual subnetworks significantly increased when synchrony frequencies were reduced by augmenting gamma-aminobutyric acid (GABA)ergic activity at P15/16, a period when the neuronal subnetworks were still maturing. Together, these findings indicate the early formation of subnetworks and underscore the pivotal roles of GABAergic inputs in modulating S1 neuronal subnetworks.

FKBP51 in glutamatergic forebrain neurons promotes early life stress inoculation in female mice

Early life stress (ELS) can increase vulnerability to psychiatric disorders, but also trigger resilience. FKBP51 has been associated with an increased risk for developing psychiatric disorders, specifically in interaction with ELS exposure. Here, the contribution of FKBP51 in glutamatergic forebrain neurons to the long-term consequences of ELS was investigated in both sexes. In female wild-type Fkbp5lox/lox mice, ELS exposure led to an anxiolytic phenotype and improved memory performance in a stressful context, however this ELS effect was absent in Fkbp5Nex mice. These interactive FKBP51 x ELS effects in female mice were also reflected in reduced brain region volumes, and on structural and electrophysiological properties of CA1 pyramidal neurons of the dorsal hippocampus. In contrast, the behavioral, structural and functional effects in male ELS mice were less pronounced and independent of FKBP51. RNA sequencing of the hippocampus revealed the transcription factor 4 (TCF4) as a potential regulator of the female interactive effects. Cre-dependent viral overexpression of TCF4 in female Nex-Cre mice led to similar beneficial effects on behavior as the ELS exposure. This study demonstrates a sex-specific role for FKBP51 in mediating the adaptive effects of ELS on emotional regulation, cognition, and neuronal function, implicating TCF4 as a downstream effector.

Dual role of FOXG1 in regulating gliogenesis in the developing neocortex via the FGF signalling pathway

In the developing vertebrate central nervous system, neurons and glia typically arise sequentially from common progenitors. Here, we report that the transcription factor Forkhead Box G1 (Foxg1) regulates gliogenesis in the mouse neocortex via distinct cell-autonomous roles in progenitors and postmitotic neurons that regulate different aspects of the gliogenic FGF signalling pathway. We demonstrate that loss of Foxg1 in cortical progenitors at neurogenic stages causes premature astrogliogenesis. We identify a novel FOXG1 target, the pro-gliogenic FGF pathway component Fgfr3, which is suppressed by FOXG1 cell-autonomously to maintain neurogenesis. Furthermore, FOXG1 can also suppress premature astrogliogenesis triggered by the augmentation of FGF signalling. We identify a second novel function of FOXG1 in regulating the expression of gliogenic cues in newborn neocortical upper-layer neurons. Loss of FOXG1 in postmitotic neurons non-autonomously enhances gliogenesis in the progenitors via FGF signalling. These results fit well with the model that newborn neurons secrete cues that trigger progenitors to produce the next wave of cell types, astrocytes. If FGF signalling is attenuated in Foxg1 null progenitors, they progress to oligodendrocyte production. Therefore, loss of FOXG1 transitions the progenitor to a gliogenic state, producing either astrocytes or oligodendrocytes depending on FGF signalling levels. Our results uncover how FOXG1 integrates extrinsic signalling via the FGF pathway to regulate the sequential generation of neurons, astrocytes, and oligodendrocytes in the cerebral cortex.

Propagation of neuronal micronuclei regulates microglial characteristics

Microglia-resident immune cells in the central nervous system-undergo morphological and functional changes in response to signals from the local environment and mature into various homeostatic states. However, niche signals underlying microglial differentiation and maturation remain unknown. Here, we show that neuronal micronuclei (MN) transfer to microglia, which is followed by changing microglial characteristics during the postnatal period. Neurons passing through a dense region of the developing neocortex give rise to MN and release them into the extracellular space, before being incorporated into microglia and inducing morphological changes. Two-photon imaging analyses have revealed that microglia incorporating MN tend to slowly retract their processes. Loss of the cGAS gene alleviates effects on micronucleus-dependent morphological changes. Neuronal MN-harboring microglia also exhibit unique transcriptome signatures. These results demonstrate that neuronal MN serve as niche signals that transform microglia, and provide a potential mechanism for regulation of microglial characteristics in the early postnatal neocortex.

The Hao-Fountain syndrome protein USP7 regulates neuronal connectivity in the brain via a novel p53-independent ubiquitin signaling pathway

Mutation or deletion of the deubiquitinase USP7 causes Hao-Fountain syndrome (HAFOUS), which is characterized by speech delay, intellectual disability, and aggressive behavior and highlights important unknown roles of USP7 in the nervous system. Here, we conditionally delete USP7 in glutamatergic neurons in the mouse forebrain, triggering disease-relevant phenotypes, including sensorimotor deficits, impaired cognition, and aggressive behavior. Although USP7 deletion induces p53-dependent neuronal apoptosis, most behavioral abnormalities in USP7 conditional knockout mice persist following p53 loss. Strikingly, USP7 deletion perturbs the synaptic proteome and dendritic spinogenesis independent of p53. Integrated proteomics and biochemical analyses identify the RNA splicing factor Ppil4 as a key substrate of USP7. Ppil4 knockdown phenocopies the effect of USP7 loss on dendritic spines. Accordingly, USP7 loss disrupts splicing of synaptic genes. These findings reveal that USP7-Ppil4 signaling regulates neuronal connectivity in the developing brain with implications for our understanding of HAFOUS pathogenesis and other neurodevelopmental disorders.

Loss of the proton-activated chloride channel in neurons impairs AMPA receptor endocytosis and LTD via endosomal hyper-acidification

Hippocampal long-term potentiation (LTP) and long-term depression (LTD) are forms of synaptic plasticity, thought to be the molecular basis of learning and memory, dependent on dynamic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) trafficking. Alteration of endosomal pH negatively affects synaptic transmission and neural development, but it is unclear how pH is involved in AMPAR trafficking. We show that the proton-activated chloride (PAC) channel localizes to early and recycling endosomes in neurons and prevents endosome hyper-acidification. Loss of PAC reduces AMPAR endocytosis during chemical LTD in primary neurons, while basal trafficking and LTP are unaffected. Pyramidal neuron-specific PAC knockout mice have impaired hippocampal LTD, but not LTP, and perform poorly in the Morris water maze reversal test, exhibiting impaired behavioral adaptation. We conclude that proper maintenance of endosomal pH by PAC in neurons is important during LTD to regulate AMPAR trafficking in a manner critical for animal physiology and behavior.

Synaptic Vesicle Glycoprotein 2A Knockout in Parvalbumin and Somatostatin Interneurons Drives Seizures in the Postnatal Mouse Brain

Synaptic vesicle glycoprotein 2A (SV2A) is a presynaptic protein targeted by the antiseizure drug levetiracetam. One or more of the three SV2 genes is expressed in all neurons and is essential to normal neurotransmission. Loss of SV2A results in a seizure phenotype in mice and mutations in humans are also linked to congenital seizures. How SV2A action impacts the epileptic phenotype remains unclear, especially among the diverse neuronal populations that regulate network excitability. This study explored how brain structure and function are affected by SV2A conditional knock-out (SV2A-cKO) in specific neural cell subtypes. We show that SV2A-cKO in all neurons of the postnatal brain triggers lethal seizures, suggesting that the seizures observed in earlier knock-out models were not due to aberrant brain development. Similar lethal seizures are detected in mice in which the loss of SV2A is limited to GABAergic neurons, whereas loss in excitatory neurons produces no noticeable phenotype. No apparent gender difference was ever observed. Further investigation revealed that SV2A-cKO in different GABAergic interneuron populations induces seizure, with variable timescales and severity. Most notably SV2A-cKO in parvalbumin interneurons (PV+) leads to lethal seizures in young animals, while SV2A-cKO in somatostatin (SST) inhibitory neurons results in seizures that were scarcely observed only in adult mice. These results support the crucial role SV2A plays in PV and SST interneurons and suggest that the action of levetiracetam may be due largely to effects on a subset of GABAergic interneurons.

Differential neurogenic patterns underlie the formation of primary and secondary areas in the developing somatosensory cortex

The cerebral cortex consists of hierarchically organized areas interconnected by reciprocal axonal projections. However, the coordination of neurogenesis to optimize neuronal production and wiring between distinct cortical areas remains largely unexplored. The somatosensory cortex plays a crucial role in processing tactile information, with inputs from peripheral sensory receptors relayed through the thalamus to the primary and secondary somatosensory areas. To investigate the dynamics of neurogenesis in cortical circuit formation, we employed temporal genetic fate mapping of glutamatergic neuron cohorts across the somatosensory cortices. Our analysis revealed that neuronal production in the secondary somatosensory cortex (S2) precedes that of the primary somatosensory cortex (S1) from the deep-layer neuron production period and terminates earlier. We further revealed a progressive decline in upper-layer neuron output in S2, attributed to the attenuation of the apical ventricular surface, resulting in a reduced number of upper-layer neurons within S2. These findings support the existence of a protomap mechanism governing the area-specific assembly of primary and secondary areas in the developing neocortex.

mRNA stability fine-tunes gene expression in the developing cortex to control neurogenesis

RNA abundance is controlled by rates of synthesis and degradation. Although mis-regulation of RNA turnover is linked to neurodevelopmental disorders, how it contributes to cortical development is largely unknown. Here, we discover the landscape of RNA stability regulation in the cerebral cortex and demonstrate that intact RNA decay machinery is essential for corticogenesis in vivo. We use SLAM-seq to measure RNA half-lives transcriptome-wide across multiple stages of cortical development. Leveraging these data, we discover cis-acting features associated with RNA stability and probe the relationship between RNA half-life and developmental expression changes. Notably, RNAs that are up-regulated across development tend to be more stable, while down-regulated RNAs are less stable. Using compound mouse genetics, we discover CNOT3, a core component of the CCR4-NOT deadenylase complex linked to neurodevelopmental disease, is essential for cortical development. Conditional knockout of Cnot3 in neural progenitors and their progeny in the developing mouse cortex leads to severe microcephaly due to altered cell fate and p53-dependent apoptosis. Finally, we define the molecular targets of CNOT3, revealing it controls expression of poorly expressed, non-optimal mRNAs in the cortex, including cell cycle-related transcripts. Collectively, our findings demonstrate that fine-tuned control of RNA turnover is crucial for brain development.

Mef2c Controls Postnatal Callosal Axon Targeting by Regulating Sensitivity to Ephrin Repulsion

Cortical connectivity is contingent on ordered emergence of neuron subtypes followed by the formation of subtype-specific axon projections. Intracortical circuits, including long-range callosal projections, are crucial for information processing, but mechanisms of intracortical axon targeting are still unclear. We find that the transcription factor Myocyte enhancer factor 2-c (Mef2c) directs the development of somatosensory cortical (S1) layer 4 and 5 pyramidal neurons during embryogenesis. During early postnatal development, Mef2c expression shifts to layer 2/3 callosal projection neurons (L2/3 CPNs), and we find a novel function for Mef2c in targeting homotopic contralateral cortical regions by S1-L2/3 CPNs. We demonstrate, using functional manipulation of EphA-EphrinA signaling in Mef2c-mutant CPNs, that Mef2c downregulates EphA6 to desensitize S1-L2/3 CPN axons to EphrinA5-repulsion at their contralateral targets. Our work uncovers dual roles for Mef2c in cortical development: regulation of laminar subtype specification during embryogenesis, and axon targeting in postnatal callosal neurons.

Bi-allelic variants in WDR47 cause a complex neurodevelopmental syndrome

Brain development requires the coordinated growth of structures and cues that are essential for forming neural circuits and cognitive functions. The corpus callosum, the largest interhemispheric connection, is formed by the axons of callosal projection neurons through a series of tightly regulated cellular events, including neuronal specification, migration, axon extension and branching. Defects in any of those steps can lead to a range of disorders known as syndromic corpus callosum dysgenesis (CCD). We report five unrelated families carrying bi-allelic variants in WDR47 presenting with CCD together with other neuroanatomical phenotypes such as microcephaly and enlarged ventricles. Using in vitro and in vivo mouse models and complementation assays, we show that WDR47 is required for survival of callosal neurons by contributing to the maintenance of mitochondrial and microtubule homeostasis. We further propose that severity of the CCD phenotype is determined by the degree of the loss of function caused by the human variants. Taken together, we identify WDR47 as a causative gene of a new neurodevelopmental syndrome characterized by corpus callosum abnormalities and other neuroanatomical malformations.

Genetic Tools in Rodents to Study Cannabinoid Functions

During the past 30 years, the endocannabinoid system (ECS) has emerged as a major signalling system in the mammalian brain regulating neurotransmission in numerous brain regions and in various cell populations. Endocannabinoids are able to regulate specific physiological functions and thus modify their behavioural manifestations and allostatic alterations of the ECS linked to different pathological conditions. As discussed in detail in other chapters of this book, endocannabinoids are involved in learning and memory, stress, and anxiety, feeding, energy balance, development, and ageing. Likewise, many CNS disorders (e.g. schizophrenia, epilepsy, substance use disorders, and multiple sclerosis) are associated with dysregulation of the ECS. Discerning the physiological functions of the synthetic and degrading enzymes of endocannabinoids and their receptors is a challenging task because of their distinct and complex expression patterns. Techniques of genetic engineering have been able to shed light on a number of complex ECS-related tasks during the past years. In this chapter, first, we take a critical look at the toolbox available to researchers who would like to investigate cannabinoid effects using genetic engineering techniques, then we comprehensively discuss genetically modified rodent models in various neuronal and non-neuronal cell populations, both within and outside the nervous system.

A novel monomeric amyloid β-activated signaling pathway regulates brain development via inhibition of microglia

Amyloid β (Aβ) forms aggregates in the Alzheimer's disease brain and is well known for its pathological roles. Recent studies show that it also regulates neuronal physiology in the healthy brain. Whether Aβ also regulates glial physiology in the normal brain, however, has remained unclear. In this article, we describe the discovery of a novel signaling pathway activated by the monomeric form of Aβ in vitro that plays essential roles in the regulation of microglial activity and the assembly of neocortex during mouse development in vivo. We find that activation of this pathway depends on the function of amyloid precursor and the heterotrimeric G protein regulator Ric8a in microglia and inhibits microglial immune activation at transcriptional and post-transcriptional levels. Genetic disruption of this pathway during neocortical development results in microglial dysregulation and excessive matrix proteinase activation, leading to basement membrane degradation, neuronal ectopia, and laminar disruption. These results uncover a previously unknown function of Aβ as a negative regulator of brain microglia and substantially elucidate the underlying molecular mechanisms. Considering the prominence of Aβ and neuroinflammation in the pathology of Alzheimer's disease, they also highlight a potentially overlooked role of Aβ monomer depletion in the development of the disease.

Postsynaptic competition between calcineurin and PKA regulates mammalian sleep-wake cycles

The phosphorylation of synaptic proteins is a significant biochemical reaction that controls the sleep-wake cycle in mammals1-3. Protein phosphorylation in vivo is reversibly regulated by kinases and phosphatases. In this study, we investigate a pair of kinases and phosphatases that reciprocally regulate sleep duration. First, we perform a comprehensive screen of protein kinase A (PKA) and phosphoprotein phosphatase (PPP) family genes by generating 40 gene knockout mouse lines using prenatal and postnatal CRISPR targeting. We identify a regulatory subunit of PKA (Prkar2b), a regulatory subunit of protein phosphatase 1 (PP1; Pppr1r9b) and catalytic and regulatory subunits of calcineurin (also known as PP2B) (Ppp3ca and Ppp3r1) as sleep control genes. Using adeno-associated virus (AAV)-mediated stimulation of PKA and PP1-calcineurin activities, we show that PKA is a wake-promoting kinase, whereas PP1 and calcineurin function as sleep-promoting phosphatases. The importance of these phosphatases in sleep regulation is supported by the marked changes in sleep duration associated with their increased and decreased activities, ranging from approximately 17.3 h per day (PP1 expression) to 4.3 h per day (postnatal CRISPR targeting of calcineurin). Localization signals to the excitatory post-synapse are necessary for these phosphatases to exert their sleep-promoting effects. Furthermore, the wake-promoting effect of PKA localized to the excitatory post-synapse negated the sleep-promoting effect of PP1-calcineurin. These findings indicate that PKA and PP1-calcineurin have competing functions in sleep regulation at excitatory post-synapses.

Loss of DOT1L function disrupts neuronal transcription, animal behavior, and leads to a novel neurodevelopmental disorder

Individuals with monoallelic pathogenic variants in the histone lysine methyltransferase DOT1L display global developmental delay and varying congenital anomalies. However, the impact of monoallelic loss of DOT1L remains unclear. Here, we present a largely female cohort of 11 individuals with DOT1L variants with developmental delays and dysmorphic facial features. We found that DOT1L variants include missense variants clustered in the catalytic domain, frameshift, and stop-gain variants. We demonstrate that specific variants cause loss of methyltransferase activity and therefore sought to define the effects of decreased DOT1L function. Using RNA-sequencing of cultured neurons and single nucleus RNA-sequencing of mouse cortical tissue, we found that partial Dot1l depletion causes sex-specific transcriptional responses and disrupts transcription of synaptic genes. Further, Dot1l loss alters neuron branching and expression of synaptic proteins. Lastly using zebrafish and mouse models, we found behavioral disruptions that include sex-specific deficits in mice. Overall, we define how DOT1L loss leads to neurological dysfunction by demonstrating that partial Dot1l loss impacts transcription, neuron morphology, and behavior across multiple models and systems.

Neuronal Mitochondrial Calcium Uniporter (MCU) Deficiency Is Neuroprotective in Hyperexcitability by Modulation of Metabolic Pathways and ROS Balance

Epilepsy is one of the most common neurological disorders in the world. Common epileptic drugs generally affect ion channels or neurotransmitters and prevent the emergence of seizures. However, up to a third of the patients suffer from drug-resistant epilepsy, and there is an urgent need to develop new therapeutic strategies that go beyond acute antiepileptic (antiseizure) therapies towards therapeutics that also might have effects on chronic epilepsy comorbidities such as cognitive decline and depression. The mitochondrial calcium uniporter (MCU) mediates rapid mitochondrial Ca2+ transport through the inner mitochondrial membrane. Ca2+ influx is essential for mitochondrial functions, but longer elevations of intracellular Ca2+ levels are closely associated with seizure-induced neuronal damage, which are underlying mechanisms of cognitive decline and depression. Using neuronal-specific MCU knockout mice (MCU-/-ΔN), we demonstrate that neuronal MCU deficiency reduced hippocampal excitability in vivo. Furthermore, in vitro analyses of hippocampal glioneuronal cells reveal no change in total Ca2+ levels but differences in intracellular Ca2+ handling. MCU-/-ΔN reduces ROS production, declines metabolic fluxes, and consequently prevents glioneuronal cell death. This effect was also observed under pathological conditions, such as the low magnesium culture model of seizure-like activity or excitotoxic glutamate stimulation, whereby MCU-/-ΔN reduces ROS levels and suppresses Ca2+ overload seen in WT cells. This study highlights the importance of MCU at the interface of Ca2+ handling and metabolism as a mediator of stress-related mitochondrial dysfunction, which indicates the modulation of MCU as a potential target for future antiepileptogenic therapy.

Cacna1c deficiency in forebrain glutamatergic neurons alters behavior and hippocampal plasticity in female mice

CACNA1C, coding for the α1 subunit of L-type voltage-gated calcium channel (LTCC) Cav1.2, has been associated with multiple psychiatric disorders. Clinical studies have revealed alterations in behavior as well as in brain structure and function in CACNA1C risk allele carriers. These findings are supported by rodent models of Cav1.2 deficiency, which showed increased anxiety, cognitive and social impairments as well as a shift towards active stress-coping strategies. These behavioral alterations were accompanied by functional deficits, such as reduced long-term potentiation (LTP) and an excitation/inhibition (E/I) imbalance. However, these preclinical studies are largely limited to male rodents, with few studies exploring sex-specific effects. Here, we investigated the effects of Cav1.2 deficiency in forebrain glutamatergic neurons in female conditional knockout (CKO) mice. CKO mice exhibited hyperlocomotion in a novel environment, increased anxiety-related behavior, cognitive deficits, and increased active stress-coping behavior. These behavioral alterations were neither influenced by the stage of the estrous cycle nor by the Nex/Neurod6 haploinsufficiency or Cre expression, which are intrinsically tied to the utilization of the Nex-Cre driver line for conditional inactivation of Cacna1c. In the hippocampus, Cav1.2 inactivation enhanced presynaptic paired-pulse facilitation without altering postsynaptic LTP at CA3-CA1 synapses. In addition, CA1 pyramidal neurons of female CKO mice displayed a reduction in dendritic complexity and spine density. Taken together, our findings extend the existing knowledge suggesting Cav1.2-dependent structural and functional alterations as possible mechanisms for the behavioral alterations observed in female Cav1.2-Nex mice.

The RNA-binding protein EIF4A3 promotes axon development by direct control of the cytoskeleton

The exon junction complex (EJC), nucleated by EIF4A3, is indispensable for mRNA fate and function throughout eukaryotes. We discover that EIF4A3 directly controls microtubules, independent of RNA, which is critical for neural wiring. While neuronal survival in the developing mouse cerebral cortex depends upon an intact EJC, axonal tract development requires only Eif4a3. Using human cortical organoids, we show that EIF4A3 disease mutations also impair neuronal growth, highlighting conserved functions relevant for neurodevelopmental pathology. Live imaging of growing neurons shows that EIF4A3 is essential for microtubule dynamics. Employing biochemistry and competition experiments, we demonstrate that EIF4A3 directly binds to microtubules, mutually exclusive of the EJC. Finally, in vitro reconstitution assays and rescue experiments demonstrate that EIF4A3 is sufficient to promote microtubule polymerization and that EIF4A3-microtubule association is a major contributor to axon growth. This reveals a fundamental mechanism by which neurons re-utilize core gene expression machinery to directly control the cytoskeleton.

Gain control of sensory input across polysynaptic circuitries in mouse visual cortex by a single G protein-coupled receptor type (5-HT2A)

Response gain is a crucial means by which modulatory systems control the impact of sensory input. In the visual cortex, the serotonergic 5-HT2A receptor is key in such modulation. However, due to its expression across different cell types and lack of methods that allow for specific activation, the underlying network mechanisms remain unsolved. Here we optogenetically activate endogenous G protein-coupled receptor (GPCR) signaling of a single receptor subtype in distinct mouse neocortical subpopulations in vivo. We show that photoactivation of the 5-HT2A receptor pathway in pyramidal neurons enhances firing of both excitatory neurons and interneurons, whereas 5-HT2A photoactivation in parvalbumin interneurons produces bidirectional effects. Combined photoactivation in both cell types and cortical network modelling demonstrates a conductance-driven polysynaptic mechanism that controls the gain of visual input without affecting ongoing baseline levels. Our study opens avenues to explore GPCRs neuromodulation and its impact on sensory-driven activity and ongoing neuronal dynamics.

The postnatal injection of AAV9-FOXG1 rescues corpus callosum agenesis and other brain deficits in the mouse model of FOXG1 syndrome

Heterozygous mutations in the FOXG1 gene manifest as FOXG1 syndrome, a severe neurodevelopmental disorder characterized by structural brain anomalies, including agenesis of the corpus callosum, hippocampal reduction, and myelination delays. Despite the well-defined genetic basis of FOXG1 syndrome, therapeutic interventions targeting the underlying cause of the disorder are nonexistent. In this study, we explore the therapeutic potential of adeno-associated virus 9 (AAV9)-mediated delivery of the FOXG1 gene. Remarkably, intracerebroventricular injection of AAV9-FOXG1 to Foxg1 heterozygous mouse model at the postnatal stage rescues a wide range of brain pathologies. This includes the amelioration of corpus callosum deficiencies, the restoration of dentate gyrus morphology in the hippocampus, the normalization of oligodendrocyte lineage cell numbers, and the rectification of myelination anomalies. Our findings highlight the efficacy of AAV9-based gene therapy as a viable treatment strategy for FOXG1 syndrome and potentially other neurodevelopmental disorders with similar brain malformations, asserting its therapeutic relevance in postnatal stages.

Paroxysmal dystonia results from the loss of RIM4 in Purkinje cells

Full-length RIM1 and 2 are key components of the presynaptic active zone that ubiquitously control excitatory and inhibitory neurotransmitter release. Here, we report that the function of the small RIM isoform RIM4, consisting of a single C2 domain, is strikingly different from that of the long isoforms. RIM4 is dispensable for neurotransmitter release but plays a postsynaptic, cell type-specific role in cerebellar Purkinje cells that is essential for normal motor function. In the absence of RIM4, Purkinje cell intrinsic firing is reduced and caffeine-sensitive, and dendritic integration of climbing fibre input is disturbed. Mice lacking RIM4, but not mice lacking RIM1/2, selectively in Purkinje cells exhibit a severe, hours-long paroxysmal dystonia. These episodes can also be induced by caffeine, ethanol or stress and closely resemble the deficits seen with mutations of the PNKD (paroxysmal non-kinesigenic dystonia) gene. Our data reveal essential postsynaptic functions of RIM proteins and show non-overlapping specialized functions of a small isoform despite high homology to a single domain in the full-length proteins.

The SWELL1 Channel Promotes Ischemic Brain Damage by Mediating Neuronal Swelling and Glutamate Toxicity

Cytotoxic neuronal swelling and glutamate excitotoxicity are two hallmarks of ischemic stroke. However, the underlying molecular mechanisms are not well understood. Here, it is reported that SWELL1, the essential subunit of the volume-regulated anion channel (VRAC), plays a dual role in ischemic injury by promoting neuronal swelling and glutamate excitotoxicity. SWELL1 expression is upregulated in neurons and astrocytes after experimental stroke in mice. The neuronal SWELL1 channel is activated by intracellular hypertonicity, leading to Cl- influx-dependent cytotoxic neuronal swelling and subsequent cell death. Additionally, the SWELL1 channel in astrocytes mediates pathological glutamate release, indicated by increases in neuronal slow inward current frequency and tonic NMDAR current. Pharmacologically, targeting VRAC with a new inhibitor, an FDA-approved drug Dicumarol, attenuated cytotoxic neuronal swelling and cell death, reduced astrocytic glutamate release, and provided significant neuroprotection in mice when administered either before or after ischemia. Therefore, these findings uncover the pleiotropic effects of the SWELL1 channel in neurons and astrocytes in the pathogenesis of ischemic stroke and provide proof of concept for therapeutically targeting it in this disease.

Oligodendrocytes produce amyloid-β and contribute to plaque formation alongside neurons in Alzheimer's disease model mice

Amyloid-β (Aβ) is thought to be neuronally derived in Alzheimer's disease (AD). However, transcripts of amyloid precursor protein (APP) and amyloidogenic enzymes are equally abundant in oligodendrocytes (OLs). By cell-type-specific deletion of Bace1 in a humanized knock-in AD model, APPNLGF, we demonstrate that OLs and neurons contribute to Aβ plaque burden. For rapid plaque seeding, excitatory projection neurons must provide a threshold level of Aβ. Ultimately, our findings are relevant for AD prevention and therapeutic strategies.

Semaphorin heterodimerization in cis regulates membrane targeting and neocortical wiring

Disruption of neocortical circuitry and architecture in humans causes numerous neurodevelopmental disorders. Neocortical cytoarchitecture is orchestrated by various transcription factors such as Satb2 that control target genes during strict time windows. In humans, mutations of SATB2 cause SATB2 Associated Syndrome (SAS), a multisymptomatic syndrome involving epilepsy, intellectual disability, speech delay, and craniofacial defects. Here we show that Satb2 controls neuronal migration and callosal axonal outgrowth during murine neocortical development by inducing the expression of the GPI-anchored protein, Semaphorin 7A (Sema7A). We find that Sema7A exerts this biological activity by heterodimerizing in cis with the transmembrane semaphorin, Sema4D. We could also observe that heterodimerization with Sema7A promotes targeting of Sema4D to the plasma membrane in vitro. Finally, we report an epilepsy-associated de novo mutation in Sema4D (Q497P) that inhibits normal glycosylation and plasma membrane localization of Sema4D-associated complexes. These results suggest that neuronal use of semaphorins during neocortical development is heteromeric, and a greater signaling complexity exists than was previously thought.

Epistatic interactions between NMD and TRP53 control progenitor cell maintenance and brain size

Mutations in human nonsense-mediated mRNA decay (NMD) factors are enriched in neurodevelopmental disorders. We show that deletion of key NMD factor Upf2 in mouse embryonic neural progenitor cells causes perinatal microcephaly but deletion in immature neurons does not, indicating NMD's critical roles in progenitors. Upf2 knockout (KO) prolongs the cell cycle of radial glia progenitor cells, promotes their transition into intermediate progenitors, and leads to reduced upper-layer neurons. CRISPRi screening identified Trp53 knockdown rescuing Upf2KO progenitors without globally reversing NMD inhibition, implying marginal contributions of most NMD targets to the cell cycle defect. Integrated functional genomics shows that NMD degrades selective TRP53 downstream targets, including Cdkn1a, which, without NMD suppression, slow the cell cycle. Trp53KO restores the progenitor cell pool and rescues the microcephaly of Upf2KO mice. Therefore, one physiological role of NMD in the developing brain is to degrade selective TRP53 targets to control progenitor cell cycle and brain size.

The Hao-Fountain syndrome protein USP7 regulates neuronal connectivity in the brain via a novel p53-independent ubiquitin signaling pathway

Precise control of protein ubiquitination is essential for brain development, and hence, disruption of ubiquitin signaling networks can lead to neurological disorders. Mutations of the deubiquitinase USP7 cause the Hao-Fountain syndrome (HAFOUS), characterized by developmental delay, intellectual disability, autism, and aggressive behavior. Here, we report that conditional deletion of USP7 in excitatory neurons in the mouse forebrain triggers diverse phenotypes including sensorimotor deficits, learning and memory impairment, and aggressive behavior, resembling clinical features of HAFOUS. USP7 deletion induces neuronal apoptosis in a manner dependent of the tumor suppressor p53. However, most behavioral abnormalities in USP7 conditional mice persist despite p53 loss. Strikingly, USP7 deletion in the brain perturbs the synaptic proteome and dendritic spine morphogenesis independently of p53. Integrated proteomics analysis reveals that the neuronal USP7 interactome is enriched for proteins implicated in neurodevelopmental disorders and specifically identifies the RNA splicing factor Ppil4 as a novel neuronal substrate of USP7. Knockdown of Ppil4 in cortical neurons impairs dendritic spine morphogenesis, phenocopying the effect of USP7 loss on dendritic spines. These findings reveal a novel USP7-Ppil4 ubiquitin signaling link that regulates neuronal connectivity in the developing brain, with implications for our understanding of the pathogenesis of HAFOUS and other neurodevelopmental disorders.

Temporal patterning of the vertebrate developing neural tube

The chronologically ordered generation of distinct cell types is essential for the establishment of neuronal diversity and the formation of neuronal circuits. Recently, single-cell transcriptomic analyses of various areas of the developing vertebrate nervous system have provided evidence for the existence of a shared temporal patterning program that partitions neurons based on the timing of neurogenesis. In this review, I summarize the findings that lead to the proposal of this shared temporal program before focusing on the developing spinal cord to discuss how temporal patterning in general and this program specifically contributes to the ordered formation of neuronal circuits.

Hyperactivation of MEK1 in cortical glutamatergic neurons results in projection axon deficits and aberrant motor learning

Abnormal extracellular signal-regulated kinase 1/2 (ERK1/2, encoded by Mapk3 and Mapk1, respectively) signaling is linked to multiple neurodevelopmental diseases, especially the RASopathies, which typically exhibit ERK1/2 hyperactivation in neurons and non-neuronal cells. To better understand how excitatory neuron-autonomous ERK1/2 activity regulates forebrain development, we conditionally expressed a hyperactive MEK1 (MAP2K1) mutant, MEK1S217/221E, in cortical excitatory neurons of mice. MEK1S217/221E expression led to persistent hyperactivation of ERK1/2 in cortical axons, but not in soma/nuclei. We noted reduced axonal arborization in multiple target domains in mutant mice and reduced the levels of the activity-dependent protein ARC. These changes did not lead to deficits in voluntary locomotion or accelerating rotarod performance. However, skilled motor learning in a single-pellet retrieval task was significantly diminished in these MEK1S217/221E mutants. Restriction of MEK1S217/221E expression to layer V cortical neurons recapitulated axonal outgrowth deficits but did not affect motor learning. These results suggest that cortical excitatory neuron-autonomous hyperactivation of MEK1 is sufficient to drive deficits in axon outgrowth, which coincide with reduced ARC expression, and deficits in skilled motor learning. Our data indicate that neuron-autonomous decreases in long-range axonal outgrowth may be a key aspect of neuropathogenesis in RASopathies.

Neuronal deletion of Gtf2i results in developmental microglial alterations in a mouse model related to Williams syndrome

Williams syndrome (WS) is a genetic neurodevelopmental disorder caused by a heterozygous microdeletion, characterized by hypersociability and unique neurocognitive abnormalities. Of the deleted genes, GTF2I has been linked to hypersociability in WS. We have recently shown that Gtf2i deletion from forebrain excitatory neurons, referred to as Gtf2i conditional knockout (cKO) mice leads to multi-faceted myelination deficits associated with the social behaviors affected in WS. These deficits were potentially mediated also by microglia, as they present a close relationship with oligodendrocytes. To study the impact of altered myelination, we characterized these mice in terms of microglia over the course of development. In postnatal day 30 (P30) Gtf2i cKO mice, cortical microglia displayed a more ramified state, as compared with wild type (controls). However, postnatal day 4 (P4) microglia exhibited high proliferation rates and an elevated activation state, demonstrating altered properties related to activation and inflammation in Gtf2i cKO mice compared with control. Intriguingly, P4 Gtf2i cKO-derived microglial cells exhibited significantly elevated myelin phagocytosis in vitro compared to control mice. Lastly, systemic injection of clemastine to P4 Gtf2i cKO and control mice until P30, led to a significant interaction between genotypes and treatments on the expression levels of the phagocytic marker CD68, and a significant reduction of the macrophage/microglial marker Iba1 transcript levels in the cortex of the Gtf2i cKO treated mice. Our data thus implicate microglia as important players in WS, and that early postnatal manipulation of microglia might be beneficial in treating inflammatory and myelin-related pathologies.

Cell type-specific expression, regulation and compensation of CDKL5 activity in mouse brain

CDKL5 is a brain-enriched serine/threonine kinase, associated with a profound developmental and epileptic encephalopathy called CDKL5 deficiency disorder (CDD). To design targeted therapies for CDD, it is essential to determine where CDKL5 is expressed and is active in the brain and test if compensatory mechanisms exist at cellular level. We generated conditional Cdkl5 knockout mice in excitatory neurons, inhibitory neurons and astrocytes. To assess CDKL5 activity, we utilized a phosphospecific antibody for phosphorylated EB2, a well-known substrate of CDKL5. We found that CDKL5 and EB2 pS222 were prominent in excitatory and inhibitory neurons but were not detected in astrocytes. We observed that approximately 15-20% of EB2 pS222 remained in Cdkl5 knockout brains and primary neurons. Surprisingly, the remaining phosphorylation was modulated by NMDA and PP1/PP2A in neuronal CDKL5 knockout cultures, indicating the presence of a compensating kinase. Using a screen of candidate kinases with highest homology to the CDKL5 kinase domain, we found that CDKL2 and ICK can phosphorylate EB2 S222 in HEK293T cells and in primary neurons. We then generated Cdkl5/Cdkl2 dual knockout mice to directly test if CDKL2 phosphorylates EB2 in vivo and found that CDKL2 phosphorylates CDKL5 substrates in the brain. This study is the first indication that CDKL2 could potentially replace CDKL5 functions in the brain, alluding to novel therapeutic possibilities.

Agrin-Lrp4 pathway in hippocampal astrocytes restrains development of temporal lobe epilepsy through adenosine signaling

BACKGROUND

Human patients often experience an episode of serious seizure activity, such as status epilepticus (SE), prior to the onset of temporal lobe epilepsy (TLE), suggesting that SE can trigger the development of epilepsy. Yet, the underlying mechanisms are not fully understood. The low-density lipoprotein receptor related protein (Lrp4), a receptor for proteoglycan-agrin, has been indicated to modulate seizure susceptibility. However, whether agrin-Lrp4 pathway also plays a role in the development of SE-induced TLE is not clear.

METHODS

Lrp4f/f mice were crossed with hGFAP-Cre and Nex-Cre mice to generate brain conditional Lrp4 knockout mice (hGFAP-Lrp4-/-) and pyramidal neuron specific knockout mice (Nex-Lrp4-/-). Lrp4 was specifically knocked down in hippocampal astrocytes by injecting AAV virus carrying hGFAP-Cre into the hippocampus. The effects of agrin-Lrp4 pathway on the development of SE-induced TLE were evaluated on the chronic seizure model generated by injecting kainic acid (KA) into the amygdala. The spontaneous recurrent seizures (SRS) in mice were video monitored.

RESULTS

We found that Lrp4 deletion from the brain but not from the pyramidal neurons elevated the seizure threshold and reduced SRS numbers, with no change in the stage or duration of SRS. More importantly, knockdown of Lrp4 in the hippocampal astrocytes after SE induction decreased SRS numbers. In accord, direct injection of agrin into the lateral ventricle of control mice but not mice with Lrp4 deletion in hippocampal astrocytes also increased the SRS numbers. These results indicate a promoting effect of agrin-Lrp4 signaling in hippocampal astrocytes on the development of SE-induced TLE. Last, we observed that knockdown of Lrp4 in hippocampal astrocytes increased the extracellular adenosine levels in the hippocampus 2 weeks after SE induction. Blockade of adenosine A1 receptor in the hippocampus by DPCPX after SE induction diminished the effects of Lrp4 on the development of SE-induced TLE.

CONCLUSION

These results demonstrate a promoting role of agrin-Lrp4 signaling in hippocampal astrocytes in the development of SE-induced development of epilepsy through elevating adenosine levels. Targeting agrin-Lrp4 signaling may serve as a potential therapeutic intervention strategy to treat TLE.

KAT6A deficiency impairs cognitive functions through suppressing RSPO2/Wnt signaling in hippocampal CA3

Intellectual disability (ID) affects ~2% of the population and ID-associated genes are enriched for epigenetic factors, including those encoding the largest family of histone lysine acetyltransferases (KAT5-KAT8). Among them is KAT6A, whose mutations cause KAT6A syndrome, with ID as a common clinical feature. However, the underlying molecular mechanism remains unknown. Here, we find that KAT6A deficiency impairs synaptic structure and plasticity in hippocampal CA3, but not in CA1 region, resulting in memory deficits in mice. We further identify a CA3-enriched gene Rspo2, encoding Wnt activator R-spondin 2, as a key transcriptional target of KAT6A. Deletion of Rspo2 in excitatory neurons impairs memory formation, and restoring RSPO2 expression in CA3 neurons rescues the deficits in Wnt signaling and learning-associated behaviors in Kat6a mutant mice. Collectively, our results demonstrate that KAT6A-RSPO2-Wnt signaling plays a critical role in regulating hippocampal CA3 synaptic plasticity and cognitive function, providing potential therapeutic targets for KAT6A syndrome and related neurodevelopmental diseases.

Activation of feedforward wiring in adult hippocampal neurons by the basic-helix-loop-helix transcription factor Ascl4

Although evidence indicates that the adult brain retains a considerable capacity for circuit formation, adult wiring has not been broadly considered and remains poorly understood. In this study, we investigate wiring activation in adult neurons. We show that the basic-helix-loop-helix transcription factor Ascl4 can induce wiring in different types of hippocampal neurons of adult mice. The new axons are mainly feedforward and reconfigure synaptic weights in the circuit. Mice with the Ascl4-induced circuits do not display signs of pathology and solve spatial problems equally well as controls. Our results demonstrate reprogrammed connectivity by a single transcriptional factor and provide insights into the regulation of brain wiring in adults.

Electroacupuncture Enhances the Functional Connectivity of Limbic System to Neocortex in the 5xFAD Mouse Model of Alzheimer's Disease

Our previous study revealed that acupuncture may exhibit therapeutic effects on Alzheimer's disease (AD) through the activation of metabolism in memory-related brain regions. However, the underlying functional mechanism remains poorly understood and warrants further investigation. In this study, we used resting-state functional magnetic resonance imaging (rsfMRI) to explore the potential effect of electroacupuncture (EA) on the 5xFAD mouse model of AD. We found that the EA group exhibited significant improvements in the number of platforms crossed and the time spent in the target quadrant when compared with the Model group (p < 0.05). The functional connectivity (FC) of left hippocampus (Hip) was enhanced significantly among 12 regions of interest (ROIs) in the EA group (p < 0.05). Based on the left Hip as the seed point, the rsfMRI analysis of the entire brain revealed increased FC between the limbic system and the neocortex in the 5xFAD mice after EA treatment. Additionally, the expression of amyloid-β(Aβ) protein and deposition in the Hip showed a downward trend in the EA group compared to the Model group (p < 0.05). In conclusion, our findings indicate that EA treatment can improve the learning and memory abilities and inhibit the expression of Aβ protein and deposition of 5xFAD mice. This improvement may be attributed to the enhancement of the resting-state functional activity and connectivity within the limbic-neocortical neural circuit, which are crucial for cognition, motor function, as well as spatial learning and memory abilities in AD mice.

Integrin α3 is required for high-frequency repetitive transcranial magnetic stimulation-induced glutamatergic synaptic transmission in mice with ischemia

BACKGROUND

Repetitive transcranial magnetic stimulation (rTMS) is an effective therapy in post-stroke motor recovery. However, the underlying mechanisms of rTMS regulates long-lasting changes with synaptic transmission and glutamate receptors function (including AMPARs or NMDARs) remains unclear.

METHODS

Mice were received 10-Hz rTMS treatment once daily on the third day after photothrombotic (PT) stroke for 18 days. Motor behaviors and the Western blot were used to evaluate the therapeutic efficacy of 10-Hz rTMS in the mice with PT model. Moreover, we used wild-type (WT) and NEX-α3-/- mice to further explore the 10-Hz rTMS effect.

RESULTS

We found that 10-Hz rTMS improved the post-stroke motor performance in the PT mice. Moreover, the levels of AMPAR, vGlut1, and integrin α3 in the peri-infarct were significantly increased in the rTMS group. In contrast, 10-Hz rTMS did not induce these aforementioned effects in NEX-α3-/- mice. The amplitude of AMPAR-mediated miniature excitatory postsynaptic currents (EPSCs) and evoked EPSCs was increased in the WT + rTMS group, but did not change in NEX-α3-/- mice with rTMS.

CONCLUSIONS

In this study, 10-Hz rTMS improved the glutamatergic synaptic transmission in the peri-infract cortex through effects on integrin α3 and AMPARs, which resulted in motor function recovery after stroke.

Differential depletion of GluN2A induces heterogeneous schizophrenia-related phenotypes in mice

BACKGROUND

Schizophrenia, a debilitating psychiatric disorder, displays considerable interindividual variation in clinical presentations. The ongoing debate revolves around whether this heterogeneity signifies a continuum of severity linked to a singular causative factor or a collection of distinct subtypes with unique origins. Within the realm of schizophrenia, the functional impairment of GluN2A, a subtype of the NMDA receptor, has been associated with an elevated risk. Despite GluN2A's expression across various neuronal types throughout the brain, its specific contributions to schizophrenia and its involvement in particular cell types or brain regions remain unexplored.

METHODS

We generated age-specific, cell type-specific or brain region-specific conditional knockout mice targeting GluN2A and conducted a comprehensive analysis using tests measuring phenotypes relevant to schizophrenia.

FINDINGS

Through the induction of germline ablation of GluN2A, we observed the emergence of numerous schizophrenia-associated abnormalities in adult mice. Intriguingly, GluN2A knockout performed at different ages, in specific cell types and within distinct brain regions, we observed overlapping yet distinct schizophrenia-related phenotypes in mice.

INTERPRETATION

Our interpretation suggests that the dysfunction of GluN2A is sufficient to evoke heterogeneous manifestations associated with schizophrenia, indicating that GluN2A stands as a prominent risk factor and a potential therapeutic target for schizophrenia.

FUNDING

This project received support from the Shanghai Municipal Science and Technology Major Project (Grant No. 2019SHZDZX02) awarded to Y.C. and the Natural Science Foundation of Shanghai (Grant No. 19ZR1468600 and 201409003800) awarded to G.Y.

ESCRT-I protein UBAP1 controls ventricular expansion and cortical neurogenesis via modulating adherens junctions of radial glial cells

Intricate cerebral cortex formation is orchestrated by the precise behavior and division dynamics of radial glial cells (RGCs). Endocytosis functions in the recycling and remodeling of adherens junctions (AJs) in response to changes in RGC activity and function. Here, we show that conditional disruption of ubiquitin-associated protein 1 (UBAP1), a component of endosomal sorting complex required for transport (ESCRT), causes severe brain dysplasia and prenatal ventriculomegaly. UBAP1 depletion disrupts the AJs and polarity of RGCs, leading to failure of apically directed interkinetic nuclear migration. Accordingly, UBAP1 knockout or knockdown results in reduced proliferation and precocious differentiation of neural progenitor cells. Mechanistically, UBAP1 regulates the expression and surface localization of cell adhesion molecules, and β-catenin over-expression significantly rescues the phenotypes of Ubap1 knockdown in vivo. Our study reveals a critical physiological role of the ESCRT machinery in cortical neurogenesis by regulating AJs of RGCs.

The AMPK-related kinase NUAK1 controls cortical axons branching by locally modulating mitochondrial metabolic functions

The cellular mechanisms underlying axonal morphogenesis are essential to the formation of functional neuronal networks. We previously identified the autism-linked kinase NUAK1 as a central regulator of axon branching through the control of mitochondria trafficking. However, (1) the relationship between mitochondrial position, function and axon branching and (2) the downstream effectors whereby NUAK1 regulates axon branching remain unknown. Here, we report that mitochondria recruitment to synaptic boutons supports collateral branches stabilization rather than formation in mouse cortical neurons. NUAK1 deficiency significantly impairs mitochondrial metabolism and axonal ATP concentration, and upregulation of mitochondrial function is sufficient to rescue axonal branching in NUAK1 null neurons in vitro and in vivo. Finally, we found that NUAK1 regulates axon branching through the mitochondria-targeted microprotein BRAWNIN. Our results demonstrate that NUAK1 exerts a dual function during axon branching through its ability to control mitochondrial distribution and metabolic activity.

Combinatorial expression of γ-protocadherins regulates synaptic connectivity in the mouse neocortex

In the process of synaptic formation, neurons must not only adhere to specific principles when selecting synaptic partners but also possess mechanisms to avoid undesirable connections. Yet, the strategies employed to prevent unwarranted associations have remained largely unknown. In our study, we have identified the pivotal role of combinatorial clustered protocadherin gamma (γ-PCDH) expression in orchestrating synaptic connectivity in the mouse neocortex. Through 5' end single-cell sequencing, we unveiled the intricate combinatorial expression patterns of γ-PCDH variable isoforms within neocortical neurons. Furthermore, our whole-cell patch-clamp recordings demonstrated that as the similarity in this combinatorial pattern among neurons increased, their synaptic connectivity decreased. Our findings elucidate a sophisticated molecular mechanism governing the construction of neural networks in the mouse neocortex.

PRMT5-mediated homologous recombination repair is essential to maintain genomic integrity of neural progenitor cells

Maintaining genomic stability is a prerequisite for proliferating NPCs to ensure genetic fidelity. Though histone arginine methylation has been shown to play important roles in safeguarding genomic stability, the underlying mechanism during brain development is not fully understood. Protein arginine N-methyltransferase 5 (PRMT5) is a type II protein arginine methyltransferase that plays a role in transcriptional regulation. Here, we identify PRMT5 as a key regulator of DNA repair in response to double-strand breaks (DSBs) during NPC proliferation. Prmt5F/F; Emx1-Cre (cKO-Emx1) mice show a distinctive microcephaly phenotype, with partial loss of the dorsal medial cerebral cortex and complete loss of the corpus callosum and hippocampus. This phenotype is resulted from DSBs accumulation in the medial dorsal cortex followed by cell apoptosis. Both RNA sequencing and in vitro DNA repair analyses reveal that PRMT5 is required for DNA homologous recombination (HR) repair. PRMT5 specifically catalyzes H3R2me2s in proliferating NPCs in the developing mouse brain to enhance HR-related gene expression during DNA repair. Finally, overexpression of BRCA1 significantly rescues DSBs accumulation and cell apoptosis in PRMT5-deficient NSCs. Taken together, our results show that PRMT5 maintains genomic stability by regulating histone arginine methylation in proliferating NPCs.

NMNAT2 supports vesicular glycolysis via NAD homeostasis to fuel fast axonal transport

BACKGROUND

Bioenergetic maladaptations and axonopathy are often found in the early stages of neurodegeneration. Nicotinamide adenine dinucleotide (NAD), an essential cofactor for energy metabolism, is mainly synthesized by Nicotinamide mononucleotide adenylyl transferase 2 (NMNAT2) in CNS neurons. NMNAT2 mRNA levels are reduced in the brains of Alzheimer's, Parkinson's, and Huntington's disease. Here we addressed whether NMNAT2 is required for axonal health of cortical glutamatergic neurons, whose long-projecting axons are often vulnerable in neurodegenerative conditions. We also tested if NMNAT2 maintains axonal health by ensuring axonal ATP levels for axonal transport, critical for axonal function.

METHODS

We generated mouse and cultured neuron models to determine the impact of NMNAT2 loss from cortical glutamatergic neurons on axonal transport, energetic metabolism, and morphological integrity. In addition, we determined if exogenous NAD supplementation or inhibiting a NAD hydrolase, sterile alpha and TIR motif-containing protein 1 (SARM1), prevented axonal deficits caused by NMNAT2 loss. This study used a combination of techniques, including genetics, molecular biology, immunohistochemistry, biochemistry, fluorescent time-lapse imaging, live imaging with optical sensors, and anti-sense oligos.

RESULTS

We provide in vivo evidence that NMNAT2 in glutamatergic neurons is required for axonal survival. Using in vivo and in vitro studies, we demonstrate that NMNAT2 maintains the NAD-redox potential to provide "on-board" ATP via glycolysis to vesicular cargos in distal axons. Exogenous NAD+ supplementation to NMNAT2 KO neurons restores glycolysis and resumes fast axonal transport. Finally, we demonstrate both in vitro and in vivo that reducing the activity of SARM1, an NAD degradation enzyme, can reduce axonal transport deficits and suppress axon degeneration in NMNAT2 KO neurons.

CONCLUSION

NMNAT2 ensures axonal health by maintaining NAD redox potential in distal axons to ensure efficient vesicular glycolysis required for fast axonal transport.

Inhibitory CCK+ basket synapse defects in mouse models of dystroglycanopathy

Dystroglycan (Dag1) is a transmembrane glycoprotein that links the extracellular matrix to the actin cytoskeleton. Mutations in Dag1 or the genes required for its glycosylation result in dystroglycanopathy, a type of congenital muscular dystrophy characterized by a wide range of phenotypes including muscle weakness, brain defects, and cognitive impairment. We investigated interneuron (IN) development, synaptic function, and associated seizure susceptibility in multiple mouse models that reflect the wide phenotypic range of dystroglycanopathy neuropathology. Mice that model severe dystroglycanopathy due to forebrain deletion of Dag1 or Pomt2, which is required for Dystroglycan glycosylation, show significant impairment of CCK+/CB1R+ IN development. CCK+/CB1R+ IN axons failed to properly target the somatodendritic compartment of pyramidal neurons in the hippocampus, resulting in synaptic defects and increased seizure susceptibility. Mice lacking the intracellular domain of Dystroglycan have milder defects in CCK+/CB1R+ IN axon targeting, but exhibit dramatic changes in inhibitory synaptic function, indicating a critical postsynaptic role of this domain. In contrast, CCK+/CB1R+ IN synaptic function and seizure susceptibility was normal in mice that model mild dystroglycanopathy due to partially reduced Dystroglycan glycosylation. Collectively, these data show that inhibitory synaptic defects and elevated seizure susceptibility are hallmarks of severe dystroglycanopathy, and show that Dystroglycan plays an important role in organizing functional inhibitory synapse assembly.

Nedd4-2-dependent regulation of astrocytic Kir4.1 and Connexin43 controls neuronal network activity

Nedd4-2 is an E3 ubiquitin ligase in which missense mutation is related to familial epilepsy, indicating its critical role in regulating neuronal network activity. However, Nedd4-2 substrates involved in neuronal network function have yet to be identified. Using mouse lines lacking Nedd4-1 and Nedd4-2, we identified astrocytic channel proteins inwardly rectifying K+ channel 4.1 (Kir4.1) and Connexin43 as Nedd4-2 substrates. We found that the expression of Kir4.1 and Connexin43 is increased upon conditional deletion of Nedd4-2 in astrocytes, leading to an elevation of astrocytic membrane ion permeability and gap junction activity, with a consequent reduction of γ-oscillatory neuronal network activity. Interestingly, our biochemical data demonstrate that missense mutations found in familial epileptic patients produce gain-of-function of the Nedd4-2 gene product. Our data reveal a process of coordinated astrocytic ion channel proteostasis that controls astrocyte function and astrocyte-dependent neuronal network activity and elucidate a potential mechanism by which aberrant Nedd4-2 function leads to epilepsy.

Histone methylation mediated by NSD1 is required for the establishment and maintenance of neuronal identities

Appropriate histone modifications emerge as essential cell fate regulators of neuronal identities across neocortical areas and layers. Here we showed that NSD1, the methyltransferase for di-methylated lysine 36 of histone H3 (H3K36me2), controls both area and layer identities of the neocortex. Nsd1-ablated neocortex showed an area shift of all four primary functional regions and aberrant wiring of cortico-thalamic-cortical projections. Nsd1 conditional knockout mice displayed defects in spatial memory, motor learning, and coordination, resembling patients with the Sotos syndrome carrying NSD1 mutations. On Nsd1 loss, superficial-layer pyramidal neurons (PNs) progressively mis-expressed markers for deep-layer PNs, and PNs remained immature both morphologically and electrophysiologically. Loss of Nsd1 in postmitotic PNs causes genome-wide loss of H3K36me2 and re-distribution of DNA methylation, which accounts for diminished expression of neocortical layer specifiers but ectopic expression of non-neural genes. Together, H3K36me2 mediated by NSD1 is required for the establishment and maintenance of region- and layer-specific neocortical identities.

Neuronal Gtf2i deletion alters mitochondrial and autophagic properties

Gtf2i encodes the general transcription factor II-I (TFII-I), with peak expression during pre-natal and early post-natal brain development stages. Because these stages are critical for proper brain development, we studied at the single-cell level the consequences of Gtf2i's deletion from excitatory neurons, specifically on mitochondria. Here we show that Gtf2i's deletion resulted in abnormal morphology, disrupted mRNA related to mitochondrial fission and fusion, and altered autophagy/mitophagy protein expression. These changes align with elevated reactive oxygen species levels, illuminating Gtf2i's importance in neurons mitochondrial function. Similar mitochondrial issues were demonstrated by Gtf2i heterozygous model, mirroring the human condition in Williams syndrome (WS), and by hemizygous neuronal Gtf2i deletion model, indicating Gtf2i's dosage-sensitive role in mitochondrial regulation. Clinically relevant, we observed altered transcript levels related to mitochondria, hypoxia, and autophagy in frontal cortex tissue from WS individuals. Our study reveals mitochondrial and autophagy-related deficits shedding light on WS and other Gtf2i-related disorders.

Callosal inputs generate side-invariant receptive fields in the barrel cortex

Barrel cortex integrates contra- and ipsilateral whiskers' inputs. While contralateral inputs depend on the thalamocortical innervation, ipsilateral ones are thought to rely on callosal axons. These are more abundant in the barrel cortex region bordering with S2 and containing the row A-whiskers representation, the row lying nearest to the facial midline. Here, we ask what role this callosal axonal arrangement plays in ipsilateral tactile signaling. We found that novel object exploration with ipsilateral whiskers confines c-Fos expression within the highly callosal subregion. Targeting this area with in vivo patch-clamp recordings revealed neurons with uniquely strong ipsilateral responses dependent on the corpus callosum, as assessed by tetrodotoxin silencing and by optogenetic activation of the contralateral hemisphere. Still, in this area, stimulation of contra- or ipsilateral row A-whiskers evoked an indistinguishable response in some neurons, mostly located in layers 5/6, indicating their involvement in the midline representation of the whiskers' sensory space.

A mouse model of ATRX deficiency with cognitive deficits and autistic traits

BACKGROUND

ATRX is an ATP-dependent chromatin remodeling protein with essential roles in safeguarding genome integrity and modulating gene expression. Deficiencies in this protein cause ATR-X syndrome, a condition characterized by intellectual disability and an array of developmental abnormalities, including features of autism. Previous studies demonstrated that deleting ATRX in mouse forebrain excitatory neurons postnatally resulted in male-specific memory deficits, but no apparent autistic-like behaviours.

METHODS

We generated mice with an earlier embryonic deletion of ATRX in forebrain excitatory neurons and characterized their behaviour using a series of memory and autistic-related paradigms.

RESULTS

We found that mutant mice displayed a broader spectrum of impairments, including fear memory, decreased anxiety-like behaviour, hyperactivity, as well as self-injurious and repetitive grooming. Sex-specific alterations were also observed, including male-specific aggression, sensory gating impairments, and decreased social memory.

CONCLUSIONS

Collectively, the findings indicate that early developmental abnormalities arising from ATRX deficiency in forebrain excitatory neurons contribute to the presentation of fear memory deficits as well as autistic-like behaviours.