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

mGluR5 Tunes NGF/TrkA Signaling to Orient Spiny Stellate Neuron Dendrites Toward Thalamocortical Axons During Whisker-Barrel Map Formation

Neurons receive and integrate synaptic inputs at their dendrites, thus dendritic patterning shapes neural connectivity and behavior. Aberrant dendritogenesis is present in neurodevelopmental disorders such as Down's syndrome and autism. Abnormal glutamatergic signaling has been observed in these diseases, as has dysfunction of the metabotropic glutamate receptor 5 (mGluR5). Deleting mGluR5 in cortical glutamatergic neurons disrupted their coordinated dendritic outgrowth toward thalamocortical axons and perturbed somatosensory circuits. Here we show that mGluR5 loss-of-function disrupts dendritogenesis of cortical neurons by increasing mRNA levels of nerve growth factor (NGF) and fibroblast growth factor 10 (FGF10), in part through calcium-permeable AMPA receptors (CP-AMPARs), as the whisker-barrel map is forming. Postnatal NGF and FGF10 expression in cortical layer IV spiny stellate neurons differentially impacted dendritic patterns. Remarkably, NGF-expressing neurons exhibited dendritic patterns resembling mGluR5 knockout neurons: increased total dendritic length/complexity and reduced polarity. Furthermore, suppressing the kinase activity of TrkA, a major NGF receptor, prevents aberrant dendritic patterning in barrel cortex of mGluR5 knockout neurons. These results reveal novel roles for NGF-TrkA signaling and CP-AMPARs for proper dendritic development of cortical neurons. This is the first in vivo demonstration that cortical neuronal NGF expression modulates dendritic patterning during postnatal brain development.

Memo1-Mediated Tiling of Radial Glial Cells Facilitates Cerebral Cortical Development

Polarized, non-overlapping, regularly spaced, tiled organization of radial glial cells (RGCs) serves as a framework to generate and organize cortical neuronal columns, layers, and circuitry. Here, we show that mediator of cell motility 1 (Memo1) is a critical determinant of radial glial tiling during neocortical development. Memo1 deletion or knockdown leads to hyperbranching of RGC basal processes and disrupted RGC tiling, resulting in aberrant radial unit assembly and neuronal layering. Memo1 regulates microtubule (MT) stability necessary for RGC tiling. Memo1 deficiency leads to disrupted MT minus-end CAMSAP2 distribution, initiation of aberrant MT branching, and altered polarized trafficking of key basal domain proteins such as GPR56, and thus aberrant RGC tiling. These findings identify Memo1 as a mediator of RGC scaffold tiling, necessary to generate and organize neurons into functional ensembles in the developing cerebral cortex.

Igf1R/InsR function is required for axon extension and corpus callosum formation

One of the earliest steps during the development of the nervous system is the establishment of neuronal polarity and the formation of an axon. The intrinsic mechanisms that promote axon formation have been extensively analyzed. However, much less is known about the extrinsic signals that initiate axon formation. One of the candidates for these signals is Insulin-like growth factor 1 (Igf1) that acts through the Igf1 (Igf1R) and insulin receptors (InsR). Since Igf1R and InsR may act redundantly we analyzed conditional cortex-specific knockout mice that are deficient for both Igf1r and Insr to determine if they regulate the development of the cortex and the formation of axons in vivo. Our results show that Igf1R/InsR function is required for the normal development of the embryonic hippocampus and cingulate cortex while the lateral cortex does not show apparent defects in the Igf1r;Insr knockout. In the cingulate cortex, the number of intermediate progenitors and deep layer neurons is reduced and the corpus callosum is absent at E17. However, cortical organization and axon formation are not impaired in knockout embryos. In culture, cortical and hippocampal neurons from Igf1r;Insr knockout embryos extend an axon but the length of this axon is severely reduced. Our results indicate that Igf1R/InsR function is required for brain development in a region-specific manner and promotes axon growth but is not essential for neuronal polarization and migration in the developing brain.

Agrin-Lrp4-Ror2 signaling regulates adult hippocampal neurogenesis in mice

Adult neurogenesis in the hippocampus may represent a form of plasticity in brain functions including mood, learning and memory. However, mechanisms underlying neural stem/progenitor cells (NSPCs) proliferation are not well understood. We found that Agrin, a factor critical for neuromuscular junction formation, is elevated in the hippocampus of mice that are stimulated by enriched environment (EE). Genetic deletion of the Agrn gene in excitatory neurons decreases NSPCs proliferation and increases depressive-like behavior. Low-density lipoprotein receptor-related protein 4 (Lrp4), a receptor for Agrin, is expressed in hippocampal NSPCs and its mutation blocked basal as well as EE-induced NSPCs proliferation and maturation of newborn neurons. Finally, we show that Lrp4 interacts with and activates receptor tyrosine kinase-like orphan receptor 2 (Ror2); and Ror2 mutation impairs NSPCs proliferation. Together, these observations identify a role of Agrin-Lrp4-Ror2 signaling for adult neurogenesis, uncovering previously unexpected functions of Agrin and Lrp4 in the brain.

Impaired anandamide/palmitoylethanolamide signaling in hippocampal glutamatergic neurons alters synaptic plasticity, learning, and emotional responses

Endocannabinoid signaling via anandamide (AEA) is implicated in a variety of neuronal functions and considered a promising therapeutic target for numerous emotion-related disorders. The major AEA degrading enzyme is fatty acid amide hydrolase (FAAH). Genetic deletion and pharmacological inhibition of FAAH reduce anxiety and improve emotional responses and memory in rodents and humans. Complementarily, the mechanisms and impact of decreased AEA signaling remain to be delineated in detail. In the present study, using the Cre/loxP system combined with an adeno-associated virus (AAV)-mediated delivery system, FAAH was selectively overexpressed in hippocampal CA1-CA3 glutamatergic neurons of adult mice. This approach led to specific FAAH overexpression at the postsynaptic site of CA1-CA3 neurons, to increased FAAH enzymatic activity, and, in consequence, to decreased hippocampal levels of AEA and palmitoylethanolamide (PEA), but the levels of the second major endocannabinoid 2-arachidonoyl glycerol (2-AG) and of oleoylethanolamide (OEA) were unchanged. Electrophysiological recordings revealed an enhancement of both excitatory and inhibitory synaptic activity and of long-term potentiation (LTP). In contrast, excitatory and inhibitory long-term depression (LTD) and short-term synaptic plasticity, apparent as depolarization-induced suppression of excitation (DSE) and inhibition (DSI), remained unaltered. These changes in hippocampal synaptic activity were associated with an increase in anxiety-like behavior, and a deficit in object recognition memory and in extinction of aversive memory. This study indicates that AEA is not involved in hippocampal short-term plasticity, or eLTD and iLTD, but modulates glutamatergic transmission most likely via presynaptic sites, and that disturbances in this process impair learning and emotional responses.

Disruption of Foxg1 impairs neural plasticity leading to social and cognitive behavioral defects

The transcription factor Foxg1 is known to be continuously expressed at a high level in mature neurons in the telencephalon, but little is known about its role in neural plasticity. Mutations in human FOXG1 cause deficiencies in learning and memory and limit social ability, which is defined as FOXG1 syndrome, but its pathogenic mechanism remains unclear. To examine the role of Foxg1 in adults, we crossed Camk2a-Cre^ER with Foxg1^fl/fl mice and conditionally disrupted Foxg1 with tamoxifen in mature neurons. We found that spatial learning and memory were significantly impaired when examined by the Morris water maze test. The cKO mice also showed a significant reduction in freezing time during the contextual and cued fear conditioning test, indicating that fear conditioning memory was affected. A remarkable reduction in Schaffer-collateral long-term potentiation was also recorded. Morphologically, the dendritic arborization and spine densities of hippocampal pyramidal neurons were significantly reduced. Primary cell culture further confirmed altered dendritic complexity after Foxg1 deletion. Our results indicated that Foxg1 plays an important role in maintaining the neural plasticity, which is vital to high-grade function.

The NeuroD6 Subtype of VTA Neurons Contributes to Psychostimulant Sensitization and Behavioral Reinforcement

Reward-related behavior is complex and its dysfunction correlated with neuropsychiatric illness. Dopamine (DA) neurons of the ventral tegmental area (VTA) have long been associated with different aspects of reward function, but it remains to be disentangled how distinct VTA DA neurons contribute to the full range of behaviors ascribed to the VTA. Here, a recently identified subtype of VTA neurons molecularly defined by NeuroD6 (NEX1M) was addressed. Among all VTA DA neurons, less than 15% were identified as positive for NeuroD6. In addition to dopaminergic markers, sparse NeuroD6 neurons expressed the vesicular glutamate transporter 2 (Vglut2) gene. To achieve manipulation of NeuroD6 VTA neurons, NeuroD6(NEX)-Cre-driven mouse genetics and optogenetics were implemented. First, expression of vesicular monoamine transporter 2 (VMAT2) was ablated to disrupt dopaminergic function in NeuroD6 VTA neurons. Comparing Vmat2^{lox/lox;NEX-Cre} conditional knock-out (cKO) mice with littermate controls, it was evident that baseline locomotion, preference for sugar and ethanol, and place preference upon amphetamine-induced and cocaine-induced conditioning were similar between genotypes. However, locomotion upon repeated psychostimulant administration was significantly elevated above control levels in cKO mice. Second, optogenetic activation of NEX-Cre VTA neurons was shown to induce DA release and glutamatergic postsynaptic currents within the nucleus accumbens. Third, optogenetic stimulation of NEX-Cre VTA neurons in vivo induced significant place preference behavior, while stimulation of VTA neurons defined by Calretinin failed to cause a similar response. The results show that NeuroD6 VTA neurons exert distinct regulation over specific aspects of reward-related behavior, findings that contribute to the current understanding of VTA neurocircuitry.

Hes1 expression in mature neurons in the adult mouse brain is required for normal behaviors

Hes1 regulates the maintenance and proliferation of neural stem/progenitor cells as an essential effector of the Notch signaling pathway. Although Notch signaling is also involved in the functions of mature neurons in learning and memory and in the risk factors for mental disorders such as schizophrenia and bipolar disorder, the in-vivo role of Hes1 in mature neurons remains unknown. Here, we found that Hes1 is expressed by subsets of both excitatory and inhibitory neurons in the adult mouse brain, and that Hes1 expression is induced by neuronal stimulation. Furthermore, inactivation of Hes1 in excitatory neurons resulted in abnormal fear and anxiety behaviors concomitantly with higher neuronal excitability in the amygdala, while inactivation of Hes1 in inhibitory neurons resulted in increased sociability and perseverative tendencies. These results indicated that Hes1 is functionally important for normal behaviors not only in excitatory neurons but also in inhibitory neurons in the adult brain.

Multiple Two-Photon Targeted Whole-Cell Patch-Clamp Recordings From Monosynaptically Connected Neurons in vivo

Although we know a great deal about monosynaptic connectivity, transmission and integration in the mammalian nervous system from in vitro studies, very little is known in vivo. This is partly because it is technically difficult to evoke action potentials and simultaneously record small amplitude subthreshold responses in closely (<150 μm) located pairs of neurons. To address this, we have developed in vivo two-photon targeted multiple (2–4) whole-cell patch clamp recordings of nearby neurons in superficial cortical layers 1–3. Here, we describe a step-by-step guide to this approach in the anesthetized mouse primary somatosensory cortex, including: the design of the setup, surgery, preparation of pipettes, targeting and acquisition of multiple whole-cell recordings, as well as in vivo and post hoc histology. The procedure takes ~4 h from start of surgery to end of recording and allows examinations both into the electrophysiological features of unitary excitatory and inhibitory monosynaptic inputs during different brain states as well as the synaptic mechanisms of correlated neuronal activity.

Characterization of perinatally born glutamatergic neurons of the mouse olfactory bulb based on NeuroD6 expression reveals their resistance to sensory deprivation

During postnatal olfactory bulb (OB) neurogenesis, predetermined stem cells residing in the ventricular-subventricular zone continuously generate progenitors that migrate in the rostral migratory stream and integrate into the OB. Although the vast majority of these postnatally generated interneurons are inhibitory, a sub-fraction represents glutamatergic neurons that integrate into the superficial glomerular layer. In the present work, we demonstrate that the bHLH transcription factor NeuroD6 is specifically and transitorily expressed in the dorsal neurogenic lineage that generates glutamatergic juxtaglomerular cells (JGCs) for the OB. Using lineage tracing combined with whole brain clearing, we provide new insight into timing of generation, morphology, and connectivity of glutamatergic JGCs. Specifically, we show that all glutamatergic JGCs send complex axons with varying projection patterns into different layers of the OB. Moreover, we find that, contrary to GABAergic OB interneurons, glutamatergic JGCs survive under sensory deprivation, indicating that inhibitory and excitatory populations are differentially susceptible to environmental stimulation.

Dgcr8 knockout approaches to understand microRNA functions in vitro and in vivo

Biologic function of the majority of microRNAs (miRNAs) is still unknown. Uncovering the function of miRNAs is hurdled by redundancy among different miRNAs. The deletion of Dgcr8 leads to the deficiency in producing all canonical miRNAs, therefore, overcoming the redundancy issue. Dgcr8 knockout strategy has been instrumental in understanding the function of miRNAs in a variety of cells in vitro and in vivo. In this review, we will first give a brief introduction about miRNAs, miRNA biogenesis pathway and the role of Dgcr8 in miRNA biogenesis. We will then summarize studies performed with Dgcr8 knockout cell models with a focus on embryonic stem cells. After that, we will summarize results from various in vivo Dgcr8 knockout models. Given significant phenotypic differences in various tissues between Dgcr8 and Dicer knockout, we will also briefly review current progresses on understanding miRNA-independent functions of miRNA biogenesis factors. Finally, we will discuss the potential use of a new strategy to stably express miRNAs in Dgcr8 knockout cells. In future, Dgcr8 knockout approaches coupled with innovations in miRNA rescue strategy may provide further insights into miRNA functions in vitro and in vivo.

Neuronal deletion of Gtf2i, associated with Williams syndrome, causes behavioral and myelin alterations rescuable by a remyelinating drug

Williams syndrome (WS), caused by a heterozygous microdeletion on chromosome 7q11.23, is a neurodevelopmental disorder characterized by hypersociability and neurocognitive abnormalities. Of the deleted genes, general transcription factor IIi (Gtf2i) has been linked to hypersociability in WS, although the underlying mechanisms are poorly understood. We show that selective deletion of Gtf2i in the excitatory neurons of the forebrain caused neuroanatomical defects, fine motor deficits, increased sociability and anxiety. Unexpectedly, 70% of the genes with significantly decreased messenger RNA levels in the mutant mouse cortex are involved in myelination, and mutant mice had reduced mature oligodendrocyte cell numbers, reduced myelin thickness and impaired axonal conductivity. Restoring myelination properties with clemastine or increasing axonal conductivity rescued the behavioral deficits. The frontal cortex from patients with WS similarly showed reduced myelin thickness, mature oligodendrocyte cell numbers and mRNA levels of myelination-related genes. Our study provides molecular and cellular evidence for myelination deficits in WS linked to neuronal deletion of Gtf2i.

Glutamate-Releasing SWELL1 Channel in Astrocytes Modulates Synaptic Transmission and Promotes Brain Damage in Stroke

By releasing glutamate, astrocytes actively regulate synaptic transmission and contribute to excitotoxicity in neurological diseases. However, the mechanisms of astrocytic glutamate release have been debated. Here, we report non-vesicular release of glutamate through the glutamate-permeable volume-regulated anion channel (VRAC). Both cell swelling and receptor stimulation activated astrocytic VRAC, which requires its only obligatory subunit, Swell1. Astrocyte-specific Swell1 knockout mice exhibited impaired glutamatergic transmission due to the decreases in presynaptic release probability and ambient glutamate level. Consistently, the mutant mice displayed hippocampal-dependent learning and memory deficits. During pathological cell swelling, deletion of astrocytic Swell1 attenuated glutamate-dependent neuronal excitability and protected mice from brain damage after ischemic stroke. Our identification of a new molecular mechanism for channel-mediated glutamate release establishes a role for astrocyte-neuron interactions in both synaptic transmission and brain ischemia. It provides a rationale for targeting VRAC for the treatment of stroke and other neurological diseases associated with excitotoxicity.

Gadd45α modulates aversive learning through post-transcriptional regulation of memory-related mRNAs

Learning is essential for survival and is controlled by complex molecular mechanisms including regulation of newly synthesized mRNAs that are required to modify synaptic functions. Despite the well‐known role of RNA‐binding proteins (RBPs) in mRNA functionality, their detailed regulation during memory consolidation is poorly understood. This study focuses on the brain function of the RBP Gadd45α (growth arrest and DNA damage‐inducible protein 45 alpha, encoded by the Gadd45a gene). Here, we find that hippocampal memory and long‐term potentiation are strongly impaired in Gadd45a‐deficient mice, a phenotype accompanied by reduced levels of memory‐related mRNAs. The majority of the Gadd45α‐regulated transcripts show unusually long 3′ untranslated regions (3′UTRs) that are destabilized in Gadd45a‐deficient mice via a transcription‐independent mechanism, leading to reduced levels of the corresponding proteins in synaptosomes. Moreover, Gadd45α can bind specifically to these memory‐related mRNAs. Our study reveals a new function for extended 3′UTRs in memory consolidation and identifies Gadd45α as a novel regulator of mRNA stability.

Conditional Mutagenesis in Oligodendrocyte Lineage Cells

Cell-type-specific gene targeting with the Cre/loxP system has become an indispensable technique in experimental neuroscience, particularly for the study of late-born glial cells that make myelin. A plethora of conditional mutants and Cre-expressing mouse lines is now available to the research community that allows laboratories to readily engage in in vivo analyses of oligodendrocytes and their precursor cells. This chapter summarizes concepts and strategies in targeting myelinating glial cells in mice for mutagenesis or imaging, and provides an overview of the most important Cre driver lines successfully used in this rapidly growing field.

Altered social behavior in mice carrying a cortical Foxp2 deletion

Genetic disruptions of the forkhead box transcription factor FOXP2 in humans cause an autosomal-dominant speech and language disorder. While FOXP2 expression pattern are highly conserved, its role in specific brain areas for mammalian social behaviors remains largely unknown. Here we studied mice carrying a homozygous cortical Foxp2 deletion. The postnatal development and gross morphological architecture of mutant mice was indistinguishable from wildtype (WT) littermates. Unbiased behavioral profiling of adult mice revealed abnormalities in approach behavior towards conspecifics as well as in the reciprocal responses of WT interaction partners. Furthermore mutant mice showed alterations in acoustical parameters of ultrasonic vocalizations, which also differed in function of the social context. Cell type-specific gene expression profiling of cortical pyramidal neurons revealed aberrant regulation of genes involved in social behavior. In particular Foxp2 mutants showed the downregulation of Mint2 (Apba2), a gene involved in approach behavior in mice and autism spectrum disorder in humans. Taken together these data demonstrate that cortical Foxp2 is required for normal social behaviors in mice.

Synaptotagmin-11 mediates a vesicle trafficking pathway that is essential for development and synaptic plasticity

Synaptotagmin-11 (Syt11) is a Synaptotagmin isoform that lacks an apparent ability to bind calcium, phospholipids, or SNARE proteins. While human genetic studies have linked mutations in the Syt11 gene to schizophrenia and Parkinson's disease, the localization or physiological role of Syt11 remain unclear. We found that in neurons, Syt11 resides on abundant vesicles that differ from synaptic vesicles and resemble trafficking endosomes. These vesicles recycle via the plasma membrane in an activity-dependent manner, but their exocytosis is slow and desynchronized. Constitutive knockout mice lacking Syt11 died shortly after birth, suggesting Syt11-mediated membrane transport is required for survival. In contrast, selective ablation of Syt11 in excitatory forebrain neurons using a conditional knockout did not affect life span but impaired synaptic plasticity and memory. Syt11-deficient neurons displayed normal secretion of fast neurotransmitters and peptides but exhibited a reduction of long-term synaptic potentiation. Hence, Syt11 is an essential component of a neuronal vesicular trafficking pathway that differs from the well-characterized synaptic vesicle trafficking pathway but is also essential for life.

Distinct molecular programs regulate synapse specificity in cortical inhibitory circuits

How neuronal connections are established and organized into functional networks determines brain function. In the mammalian cerebral cortex, different classes of GABAergic interneurons exhibit specific connectivity patterns that underlie their ability to shape temporal dynamics and information processing. Much progress has been made toward parsing interneuron diversity, yet the molecular mechanisms by which interneuron-specific connectivity motifs emerge remain unclear. In this study, we investigated transcriptional dynamics in different classes of interneurons during the formation of cortical inhibitory circuits in mouse. We found that whether interneurons form synapses on the dendrites, soma, or axon initial segment of pyramidal cells is determined by synaptic molecules that are expressed in a subtype-specific manner. Thus, cell-specific molecular programs that unfold during early postnatal development underlie the connectivity patterns of cortical interneurons.

Enhanced activity of pyramidal neurons in the infralimbic cortex drives anxiety behavior

We show that in an animal model of anxiety the overall excitation, particularly in the infralimbic region of the medial prefrontal cortex (IL), is increased and that the activity ratio between excitatory pyramidal neurons and inhibitory interneurons (AR PN/IN) is shifted towards excitation. The same change in AR PN/IN is evident for wildtype mice, which have been exposed to an anxiety stimulus. We hypothesize, that an elevated activity and the imbalance of excitation (PN) and inhibition (IN) within the neuronal microcircuitry of the prefrontal cortex is responsible for anxiety behaviour and employed optogenetic methods in freely moving mice to verify our findings. Consistent with our hypothesis elevation of pyramidal neuron activity in the infralimbic region of the prefrontal cortex significantly enhanced anxiety levels in several behavioural tasks by shifting the AR PN/IN to excitation, without affecting motor behaviour, thus revealing a novel mechanism by which anxiety is facilitated.

CPEB4-Dependent Neonate-Born Granule Cells Are Required for Olfactory Discrimination

The rodent olfactory bulb (OB) contains two distinct populations of postnatally born interneurons, mainly granule cells (GCs), to support local circuits throughout life. During the early postnatal period (i.e., 2 weeks after birth), GCs are mostly produced locally from progenitor cells in the OB with a proportion of them deriving from proliferating cells in the rostral migratory stream (RMS). Afterward, the replenishment of GCs involves differentiated neuroblasts from the subventricular zone (SVZ) in a process known as adult neurogenesis. Although numerous studies have addressed the role of SVZ-born GCs in olfactory behaviors, the function of GCs produced early postnatally in the OB remains elusive. Our previous study demonstrated that the translational regulator, cytoplasmic polyadenylation element-binding protein 4 (CPEB4), is a survival factor exclusively for neonate-born but not SVZ/adult-derived GCs, so CPEB4-knockout (KO) mice provide unique leverage to study early postnatal-born GC-regulated olfactory functions. CPEB4-KO mice with hypoplastic OBs showed normal olfactory sensitivity and short-term memory, but impaired ability to spontaneously discriminate two odors. Such olfactory dysfunction was recapitulated in specific ablation of Cpeb4 gene in inhibitory interneurons but not in excitatory projection neurons or SVZ-derived interneurons. The continuous supply of GCs from adult neurogenesis eventually restored the OB size but not the discrimination function in 6-month-old KO mice. Hence, in the early postnatal OB, whose function cannot be replaced by adult-born GCs, construct critical circuits for odor discrimination.

Single-cell transcriptomic analysis of mouse neocortical development

The development of the mammalian cerebral cortex depends on careful orchestration of proliferation, maturation, and migration events, ultimately giving rise to a wide variety of neuronal and non-neuronal cell types. To better understand cellular and molecular processes that unfold during late corticogenesis, we perform single-cell RNA-seq on the mouse cerebral cortex at a progenitor driven phase (embryonic day 14.5) and at birth-after neurons from all six cortical layers are born. We identify numerous classes of neurons, progenitors, and glia, their proliferative, migratory, and activation states, and their relatedness within and across age. Using the cell-type-specific expression patterns of genes mutated in neurological and psychiatric diseases, we identify putative disease subtypes that associate with clinical phenotypes. Our study reveals the cellular template of a complex neurodevelopmental process, and provides a window into the cellular origins of brain diseases.

Local cortical circuit correlates of altered EEG in the mouse model of Fragile X syndrome

Electroencephalogram (EEG) recordings in Fragile X syndrome (FXS) patients have revealed enhanced sensory responses, enhanced resting "gamma frequency" (30-100 Hz) activity, and a decreased ability for sensory stimuli to modulate cortical activity at gamma frequencies. Similar changes are observed in the FXS model mouse - the Fmr1 knockout. These alterations may become effective biomarkers for diagnosis and treatment of FXS. Therefore, it is critical to better understand what circuit properties underlie these changes. We employed Channelrhodopsin2 to optically activate local circuits in the auditory cortical region in brain slices to examine how changes in local circuit function may be related to EEG changes. We focused on layers 2/3 and 5 (L2/3 and L5). In Fmr1 knockout mice, light-driven excitation of L2/3 revealed hyperexcitability and increased gamma frequency power in both local L2/3 and L5 circuits. Moreover, there is increased synchrony in the gamma frequency band between L2/3 and L5. Hyperexcitability and increased gamma power were not observed in L5 with L5 light-driven excitation, indicating that these changes were layer-specific. A component of L2/3 network hyperexcitability is independent of ionotropic receptor mediated synaptic transmission and may be mediated by increased intrinsic excitability of L2/3 neurons. Finally, lovastatin, a candidate therapeutic compound for FXS that targets ERK signaling did not normalize changes in gamma activity. In conclusion, hyperactivity and increased gamma activity in local neocortical circuits, together with increased gamma synchrony between circuits, provide a putative substrate for EEG alterations observed in both FXS patients and the FXS mouse model.

Meningeal Bmps Regulate Cortical Layer Formation

Neuronal connectivity in the cortex is determined by the laminar positioning of neurons. An important determinant of laminar positioning is likely to be the control of leading process behavior during migration, maintaining their tips directed toward the pia. In this study, we provide evidence that pial bone morphogenetic protein (Bmp) signaling regulates cortical neuronal migration during cortical layer formation. Specific disruption of pial Bmp ligands impaired the positioning of early-born neurons in the deep layer; further, cell-autonomous inhibition of Smad4, a core nuclear factor mediating Bmp signaling, in the cortical radial glial cells or postmitotic cortical neurons also produced neuronal migration defects that blurred the cortical layers. We found that leading processes were abnormal and that this was accompanied by excess dephosphorylated cofilin-1, an actin-severing protein, in Smad4 mutant neurons. This suggested that regulation of cofilin-1 might transduce Bmp signaling in the migrating neurons. Ectopic expression of a phosphorylation-defective form of cofilin-1 in the late-born wild-type neurons led them to stall in the deep layer, similar to the Smad4 mutant neurons. Expression of a phosphomimetic variant of cofilin-1 in the Smad4 mutant neurons rescued the migration defects. This suggests that cofilin-1 activity underlies Bmp-mediated cortical neuronal migration. This study shows that cofilin-1 mediates pial Bmp signaling during the positioning of cortical neurons and the formation of cortical layers.

FOXG1 Orchestrates Neocortical Organization and Cortico-Cortical Connections

The hallmarks of FOXG1 syndrome, which results from mutations in a single FOXG1 allele, include cortical atrophy and corpus callosum agenesis. However, the etiology for these structural deficits and the role of FOXG1 in cortical projection neurons remain unclear. Here we demonstrate that Foxg1 in pyramidal neurons plays essential roles in establishing cortical layers and the identity and axon trajectory of callosal projection neurons. The neuron-specific actions of Foxg1 are achieved by forming a transcription complex with Rp58. The Foxg1-Rp58 complex directly binds and represses Robo1, Slit3, and Reelin genes, the key regulators of callosal axon guidance and neuronal migration. We also found that inactivation of one Foxg1 allele specifically in cortical neurons was sufficient to cause cerebral cortical hypoplasia and corpus callosum agenesis. Together, this study reveals a novel gene regulatory pathway that specifies neuronal characteristics during cerebral cortex development and sheds light on the etiology of FOXG1 syndrome. VIDEO ABSTRACT.

MARCKS Is Necessary for Netrin-DCC Signaling and Corpus Callosum Formation

Axons of the corpus callosum (CC), the white matter tract that connects the left and right hemispheres of the brain, receive instruction from a number of chemoattractant and chemorepulsant cues during their initial navigation towards and across the midline. While it has long been known that the CC is malformed in the absence of Myristoylated alanine-rich C-kinase substrate (MARCKS), evidence for a direct role of MARCKS in axon navigation has been lacking. Here, we show that MARCKS is necessary for Netrin-1 (NTN1) signaling through the DCC receptor, which is critical for axon guidance decisions. Marcks null (Marcks-/-) neurons fail to respond to exogenous NTN1 and are deficient in markers of DCC activation. Without MARCKS, the subcellular distributions of two critical mediators of NTN1-DCC signaling, the tyrosine kinases PTK2 and SRC, are disrupted. Together, this work establishes a novel role for MARCKS in axon dynamics and highlights the necessity of MARCKS as an organizer of DCC signaling at the membrane.

Haploinsufficiency of autism spectrum disorder candidate gene NUAK1 impairs cortical development and behavior in mice

Recently, numerous rare de novo mutations have been identified in patients diagnosed with autism spectrum disorders (ASD). However, despite the predicted loss-of-function nature of some of these de novo mutations, the affected individuals are heterozygous carriers, which would suggest that most of these candidate genes are haploinsufficient and/or lead to expression of dominant-negative forms of the protein. Here, we tested this hypothesis with the candidate ASD gene Nuak1 that we previously identified for its role in the development of cortical connectivity. We report that Nuak1 is haploinsufficient in mice with regard to its function in cortical development. Furthermore Nuak1^+/− mice show a combination of abnormal behavioral traits ranging from defective spatial memory consolidation, defects in social novelty (but not social preference) and abnormal sensorimotor gating. Overall, our results demonstrate that Nuak1 haploinsufficiency leads to defects in the development of cortical connectivity and a complex array of behavorial deficits.

Nuak1 is an autism spectrum disorder candidate gene. Here the authors report behavioral and cortical development in mice heterozygous for Nuak1, suggesting loss of function mutations in one copy of Nuak1 may contribute to neurodevelopmental disorders.

GABA Neuronal Deletion of Shank3 Exons 14-16 in Mice Suppresses Striatal Excitatory Synaptic Input and Induces Social and Locomotor Abnormalities

Shank3 is an excitatory postsynaptic scaffolding protein implicated in multiple brain disorders, including autism spectrum disorders (ASD) and Phelan-McDermid syndrome (PMS). Although previous neurobiological studies on Shank3 and Shank3-mutant mice have revealed diverse roles of Shank3 in the regulation of synaptic, neuronal and brain functions, whether Shank3 expression in specific cell types distinctly contributes to mouse phenotypes remains largely unclear. In the present study, we generated two Shank3-mutant mouse lines (exons 14–16) carrying global and GABA neuron-specific deletions and characterized their electrophysiological and behavioral phenotypes. These mouse lines show similar decreases in excitatory synaptic input onto dorsolateral striatal neurons. In addition, the abnormal social and locomotor behaviors observed in global Shank3-mutant mice are strongly mimicked by GABA neuron-specific Shank3-mutant mice, whereas the repetitive and anxiety-like behaviors are only partially mimicked. These results suggest that GABAergic Shank3 (exons 14–16) deletion has strong influences on striatal excitatory synaptic transmission and social and locomotor behaviors in mice.

Chemical genetic identification of CDKL5 substrates reveals its role in neuronal microtubule dynamics

Loss‐of‐function mutations in CDKL5 kinase cause severe neurodevelopmental delay and early‐onset seizures. Identification of CDKL5 substrates is key to understanding its function. Using chemical genetics, we found that CDKL5 phosphorylates three microtubule‐associated proteins: MAP1S, EB2 and ARHGEF2, and determined the phosphorylation sites. Substrate phosphorylations are greatly reduced in CDKL5 knockout mice, verifying these as physiological substrates. In CDKL5 knockout mouse neurons, dendritic microtubules have longer EB3‐labelled plus‐end growth duration and these altered dynamics are rescued by reduction of MAP1S levels through shRNA expression, indicating that CDKL5 regulates microtubule dynamics via phosphorylation of MAP1S. We show that phosphorylation by CDKL5 is required for MAP1S dissociation from microtubules. Additionally, anterograde cargo trafficking is compromised in CDKL5 knockout mouse dendrites. Finally, EB2 phosphorylation is reduced in patient‐derived human neurons. Our results reveal a novel activity‐dependent molecular pathway in dendritic microtubule regulation and suggest a pathological mechanism which may contribute to CDKL5 deficiency disorder.

Role of sodium channel subtype in action potential generation by neocortical pyramidal neurons

Neocortical pyramidal neurons express several distinct subtypes of voltage-gated Na⁺ channels. In mature cells, Na_v1.6 is the dominant channel subtype in the axon initial segment (AIS) as well as in the nodes of Ranvier. Action potentials (APs) are initiated in the AIS, and it has been proposed that the high excitability of this region is related to the unique characteristics of the Na_v1.6 channel. Knockout or loss-of-function mutation of the Scn8a gene is generally lethal early in life because of the importance of this subtype in noncortical regions of the nervous system. Using the Cre/loxP system, we selectively deleted Na_v1.6 in excitatory neurons of the forebrain and characterized the excitability of Na_v1.6-deficient layer 5 pyramidal neurons by patch-clamp and Na⁺ and Ca²⁺ imaging recordings. We now report that, in the absence of Na_v1.6 expression, the AIS is occupied by Na_v1.2 channels. However, APs are generated in the AIS, and differences in AP propagation to soma and dendrites are minimal. Moreover, the channels that are expressed in the AIS still show a clear hyperpolarizing shift in voltage dependence of activation, compared with somatic channels. The only major difference between Na_v1.6-null and wild-type neurons was a strong reduction in persistent sodium current. We propose that the molecular environment of the AIS confers properties on whatever Na channel subtype is present and that some other benefit must be conferred by the selective axonal presence of the Na_v1.6 channel.

Mammalian TRIM67 Functions in Brain Development and Behavior

Class I members of the tripartite motif (TRIM) family of E3 ubiquitin ligases evolutionarily appeared just prior to the advent of neuronal like cells and have been implicated in neuronal development from invertebrates to mammals. The single Class I TRIM in Drosophila melanogaster and Caenorhabditis elegans and the mammalian Class I TRIM9 regulate axon branching and guidance in response to the guidance cue netrin, whereas mammalian TRIM46 establishes the axon initial segment. In humans, mutations in TRIM1 and TRIM18 are implicated in Opitz Syndrome, characterized by midline defects and often intellectual disability. We find that although TRIM67 is the least studied vertebrate Class I TRIM, it is the most evolutionarily conserved. Here we show that mammalian TRIM67 interacts with both its closest paralog TRIM9 and the netrin receptor DCC and is differentially enriched in specific brain regions during development and adulthood. We describe the anatomical and behavioral consequences of deletion of murine Trim67. While viable, mice lacking Trim67 exhibit abnormal anatomy of specific brain regions, including hypotrophy of the hippocampus, striatum, amygdala, and thalamus, and thinning of forebrain commissures. Additionally, Trim67^-/- mice display impairments in spatial memory, cognitive flexibility, social novelty preference, muscle function, and sensorimotor gating, whereas several other behaviors remain intact. This study demonstrates the necessity for TRIM67 in appropriate brain development and behavior.

CNS Neurons Deposit Laminin α5 to Stabilize Synapses

Synapses in the developing brain are structurally dynamic but become stable by early adulthood. We demonstrate here that an α5-subunit-containing laminin stabilizes synapses during this developmental transition. Hippocampal neurons deposit laminin α5 at synapses during adolescence as connections stabilize. Disruption of laminin α5 in neurons causes dramatic fluctuations in dendritic spine head size that can be rescued by exogenous α5-containing laminin. Conditional deletion of laminin α5 in vivo increases dendritic spine size and leads to an age-dependent loss of synapses accompanied by behavioral defects. Remaining synapses have larger postsynaptic densities and enhanced neurotransmission. Finally, we provide evidence that laminin α5 acts through an integrin α3β1-Abl2 kinase-p190RhoGAP signaling cascade and partners with laminin β2 to regulate dendritic spine density and behavior. Together, our results identify laminin α5 as a stabilizer of dendritic spines and synapses in the brain and elucidate key cellular and molecular mechanisms by which it acts.

The Epigenetic State of PRDM16-Regulated Enhancers in Radial Glia Controls Cortical Neuron Position

The epigenetic landscape is dynamically remodeled during neurogenesis. However, it is not understood how chromatin modifications in neural stem cells instruct the formation of complex structures in the brain. We report that the histone methyltransferase PRDM16 is required in radial glia to regulate lineage-autonomous and stage-specific gene expression programs that control number and position of upper layer cortical projection neurons. PRDM16 regulates the epigenetic state of transcriptional enhancers to activate genes involved in intermediate progenitor cell production and repress genes involved in cell migration. The histone methyltransferase domain of PRDM16 is necessary in radial glia to promote cortical neuron migration through transcriptional silencing. We show that repression of the gene encoding the E3 ubiquitin ligase PDZRN3 by PRDM16 determines the position of upper layer neurons. These findings provide insights into how epigenetic control of transcriptional enhancers in radial glial determines the organization of the mammalian cerebral cortex.

Chromatin Remodeling BAF155 Subunit Regulates the Genesis of Basal Progenitors in Developing Cortex

The abundance of basal progenitors (BPs), basal radial glia progenitors (bRGs) and basal intermediate progenitors (bIPs), in primate brain has been correlated to the high degree of cortical folding. Here we examined the role of BAF155, a subunit of the chromatin remodeling BAF complex, in generation of cortical progenitor heterogeneity. The conditional deletion of BAF155 led to diminished bIP pool and increased number of bRGs, due to delamination of apical RGs. We found that BAF155 is required for normal activity of neurogenic transcription factor PAX6, thus controlling the expression of genes that are involved in bIP specification, cell-cell interaction, and establishment of adherens junction. In a PAX6-dependent manner, BAF155 regulates the expression of the CDC42 effector protein CEP4, thereby controlling progenitor delamination. Furthermore, BAF155-dependent chromatin remodeling seems to exert a specific role in the genesis of BPs through the regulation of human RG-specific genes (such as Foxn4) that possibly acquired evolutionary significance.

Chronic CRH depletion from GABAergic, long-range projection neurons in the extended amygdala reduces dopamine release and increases anxiety

The interplay between corticotropin-releasing hormone (CRH) and the dopaminergic system has predominantly been studied in addiction and reward, while CRH-dopamine interactions in anxiety are scarcely understood. We describe a new population of CRH-expressing, GABAergic, long-range-projecting neurons in the extended amygdala that innervate the ventral tegmental area and alter anxiety following chronic CRH depletion. These neurons are part of a distinct CRH circuit that acts anxiolytically by positively modulating dopamine release.

Combinatorial Expression of Grp and Neurod6 Defines Dopamine Neuron Populations with Distinct Projection Patterns and Disease Vulnerability

Midbrain dopamine neurons project to numerous targets throughout the brain to modulate various behaviors and brain states. Within this small population of neurons exists significant heterogeneity based on physiology, circuitry, and disease susceptibility. Recent studies have shown that dopamine neurons can be subdivided based on gene expression; however, the extent to which genetic markers represent functionally relevant dopaminergic subpopulations has not been fully explored. Here we performed single-cell RNA-sequencing of mouse dopamine neurons and validated studies showing that Neurod6 and Grp are selective markers for dopaminergic subpopulations. Using a combination of multiplex fluorescent in situ hybridization, retrograde labeling, and electrophysiology in mice of both sexes, we defined the anatomy, projection targets, physiological properties, and disease vulnerability of dopamine neurons based on Grp and/or Neurod6 expression. We found that the combinatorial expression of Grp and Neurod6 defines dopaminergic subpopulations with unique features. Grp+/Neurod6+ dopamine neurons reside in the ventromedial VTA, send projections to the medial shell of the nucleus accumbens, and have noncanonical physiological properties. Grp+/Neurod6- dopamine neurons are found in the VTA as well as in the ventromedial portion of the SNc, where they project selectively to the dorsomedial striatum. Grp-/Neurod6+ dopamine neurons represent a smaller VTA subpopulation, which is preferentially spared in a 6-OHDA model of Parkinson's disease. Together, our work provides detailed characterization of Neurod6 and Grp expression in the midbrain and generates new insights into how these markers define functionally relevant dopaminergic subpopulations.

Pyramidal cell regulation of interneuron survival sculpts cortical networks

Complex neuronal circuitries such as those found in the mammalian cerebral cortex have evolved as balanced networks of excitatory and inhibitory neurons. Although the establishment of appropriate numbers of these cells is essential for brain function and behaviour, our understanding of this fundamental process is limited. Here we show that the survival of interneurons in mice depends on the activity of pyramidal cells in a critical window of postnatal development, during which excitatory synaptic input to individual interneurons predicts their survival or death. Pyramidal cells regulate interneuron survival through the negative modulation of PTEN signalling, which effectively drives interneuron cell death during this period. Our findings indicate that activity-dependent mechanisms dynamically adjust the number of inhibitory cells in nascent local cortical circuits, ultimately establishing the appropriate proportions of excitatory and inhibitory neurons in the cerebral cortex.

Brain region-specific disruption of Shank3 in mice reveals a dissociation for cortical and striatal circuits in autism-related behaviors

We previously reported a new line of Shank3 mutant mice which led to a complete loss of Shank3 by deleting exons 4-22 (Δe4-22) globally. Δe4-22 mice display robust ASD-like behaviors including impaired social interaction and communication, increased stereotypical behavior and excessive grooming, and a profound deficit in instrumental learning. However, the anatomical and neural circuitry underlying these behaviors are unknown. We generated mice with Shank3 selectively deleted in forebrain, striatum, and striatal D1 and D2 cells. These mice were used to interrogate the circuit/brain-region and cell-type specific role of Shank3 in the expression of autism-related behaviors. Whole-cell patch recording and biochemical analyses were used to study the synaptic function and molecular changes in specific brain regions. We found perseverative exploratory behaviors in mice with deletion of Shank3 in striatal inhibitory neurons. Conversely, self-grooming induced lesions were observed in mice with deletion of Shank3 in excitatory neurons of forebrain. However, social, communicative, and instrumental learning behaviors were largely unaffected in these mice, unlike what is seen in global Δe4-22 mice. We discovered unique patterns of change for the biochemical and electrophysiological findings in respective brain regions that reflect the complex nature of transcriptional regulation of Shank3. Reductions in Homer1b/c and membrane hyper-excitability were observed in striatal loss of Shank3. By comparison, Shank3 deletion in hippocampal neurons resulted in increased NMDAR-currents and GluN2B-containing NMDARs. These results together suggest that Shank3 may differentially regulate neural circuits that control behavior. Our study supports a dissociation of Shank3 functions in cortical and striatal neurons in ASD-related behaviors, and it illustrates the complexity of neural circuit mechanisms underlying these behaviors.

Single synaptic inputs drive high-precision action potentials in parvalbumin expressing GABA-ergic cortical neurons in vivo

A defining feature of cortical layer 2/3 excitatory neurons is their sparse activity, often firing in singlets of action potentials. Local inhibitory neurons are thought to play a major role in regulating sparseness, but which cell types are recruited by single excitatory synaptic inputs is unknown. Using multiple, targeted, in vivo whole-cell recordings, we show that single _uEPSPs have little effect on the firing rates of excitatory neurons and somatostatin-expressing GABA-ergic inhibitory neurons but evoke precisely timed action potentials in parvalbumin-expressing inhibitory neurons. Despite a _uEPSP decay time of 7.8 ms, the evoked action potentials were almost completely restricted to the _uEPSP rising phase (~0.5 ms). Evoked parvalbumin-expressing neuron action potentials go on to inhibit the local excitatory network, thus providing a pathway for single spike evoked disynaptic inhibition which may enforce sparse and precisely timed cortical signaling.

Perineuronal Net Protein Neurocan Inhibits NCAM/EphA3 Repellent Signaling in GABAergic Interneurons

Perineuronal nets (PNNs) are implicated in closure of critical periods of synaptic plasticity in the brain, but the molecular mechanisms by which PNNs regulate synapse development are obscure. A receptor complex of NCAM and EphA3 mediates postnatal remodeling of inhibitory perisomatic synapses of GABAergic interneurons onto pyramidal cells in the mouse frontal cortex necessary for excitatory/inhibitory balance. Here it is shown that enzymatic removal of PNN glycosaminoglycan chains decreased the density of GABAergic perisomatic synapses in mouse organotypic cortical slice cultures. Neurocan, a key component of PNNs, was expressed in postnatal frontal cortex in apposition to perisomatic synapses of parvalbumin-positive interneurons. Polysialylated NCAM (PSA-NCAM), which is required for ephrin-dependent synapse remodeling, bound less efficiently to neurocan than mature, non-PSA-NCAM. Neurocan bound the non-polysialylated form of NCAM at the EphA3 binding site within the immunoglobulin-2 domain. Neurocan inhibited NCAM/EphA3 association, membrane clustering of NCAM/EphA3 in cortical interneuron axons, EphA3 kinase activation, and ephrin-A5-induced growth cone collapse. These studies delineate a novel mechanism wherein neurocan inhibits NCAM/EphA3 signaling and axonal repulsion, which may terminate postnatal remodeling of interneuron axons to stabilize perisomatic synapses in vivo.

Genetic and pharmacological inhibition of Cdk1 provides neuroprotection towards ischemic neuronal death

Cell cycle proteins are mainly expressed by dividing cells. However, it is well established that these molecules play additional non-canonical activities in several cell death contexts. Increasing evidence shows expression of cell cycle regulating proteins in post-mitotic cells, including mature neurons, following neuronal insult. Several cyclin-dependent kinases (Cdks) have already been shown to mediate ischemic neuronal death but Cdk1, a major cell cycle G2/M regulator, has not been investigated in this context. We therefore examined the role of Cdk1 in neuronal cell death following cerebral ischemia, using both in vitro and in vivo genetic and pharmacological approaches. Exposure of primary cortical neurons cultures to 4 h of oxygen–glucose deprivation (OGD) resulted in neuronal cell death and induced Cdk1 expression. Neurons from Cdk1-cKO mice showed partial resistance to OGD-induced neuronal cell death. Addition of R-roscovitine to the culture medium conferred neuroprotection against OGD-induced neuronal death. Transient 1-h occlusion of the cerebral artery (MCAO) also leads to Cdk1 expression and activation. Cdk1-cKO mice displayed partial resistance to transient 1-h MCAO. Moreover, systemic delivery of R-roscovitine was neuroprotective following transient 1-h MCAO. This study demonstrates that promising neuroprotective therapies can be considered through inhibition of the cell cycle machinery and particularly through pharmacological inhibition of Cdk1.

Rap1 GTPases Are Master Regulators of Neural Cell Polarity in the Developing Neocortex

During the development of the mammalian neocortex, the generation of neurons by neural progenitors and their migration to the final position are closely coordinated. The highly polarized radial glial cells (RGCs) serve both as progenitor cells to generate neurons and as support for the migration of these neurons. After their generation, neurons transiently assume a multipolar morphology before they polarize and begin their migration along the RGCs. Here, we show that Rap1 GTPases perform essential functions for cortical organization as master regulators of cell polarity. Conditional deletion of Rap1 GTPases leads to a complete loss of cortical lamination. In RGCs, Rap1 GTPases are required to maintain their polarized organization. In newborn neurons, the loss of Rap1 GTPases prevents the formation of axons and leading processes and thereby interferes with radial migration. Taken together, the loss of RGC and neuronal polarity results in the disruption of cortical organization.

Cross-disorder risk gene CACNA1C differentially modulates susceptibility to psychiatric disorders during development and adulthood

Single-nucleotide polymorphisms (SNPs) in CACNA1C, the α1C subunit of the voltage-gated L-type calcium channel Cav1.2, rank among the most consistent and replicable genetics findings in psychiatry and have been associated with schizophrenia, bipolar disorder and major depression. However, genetic variants of complex diseases often only confer a marginal increase in disease risk, which is additionally influenced by the environment. Here we show that embryonic deletion of Cacna1c in forebrain glutamatergic neurons promotes the manifestation of endophenotypes related to psychiatric disorders including cognitive decline, impaired synaptic plasticity, reduced sociability, hyperactivity and increased anxiety. Additional analyses revealed that depletion of Cacna1c during embryonic development also increases the susceptibility to chronic stress, which suggest that Cav1.2 interacts with the environment to shape disease vulnerability. Remarkably, this was not observed when Cacna1c was deleted in glutamatergic neurons during adulthood, where the later deletion even improved cognitive flexibility, strengthened synaptic plasticity and induced stress resilience. In a parallel gene × environment design in humans, we additionally demonstrate that SNPs in CACNA1C significantly interact with adverse life events to alter the risk to develop symptoms of psychiatric disorders. Overall, our results further validate Cacna1c as a cross-disorder risk gene in mice and humans, and additionally suggest a differential role for Cav1.2 during development and adulthood in shaping cognition, sociability, emotional behavior and stress susceptibility. This may prompt the consideration for pharmacological manipulation of Cav1.2 in neuropsychiatric disorders with developmental and/or stress-related origins.

Controlling of glutamate release by neuregulin3 via inhibiting the assembly of the SNARE complex

Neuregulin3 (NRG3) is a growth factor of the neuregulin (NRG) family and a risk gene of various severe mental illnesses including schizophrenia, bipolar disorders, and major depression. However, the physiological function of NRG3 remains poorly understood. Here we show that loss of Nrg3 in GFAP-Nrg3^f/f mice increased glutamatergic transmission, but had no effect on GABAergic transmission. These phenotypes were observed in Nex-Nrg3^f/f mice, where Nrg3 was specifically knocked out in pyramidal neurons, indicating that Nrg3 regulates glutamatergic transmission by a cell-autonomous mechanism. Consequently, in the absence of Nrg3 in pyramidal neurons, mutant mice displayed various behavioral deficits related to mental illnesses. We show that the Nrg3 mutation decreased paired-pulse facilitation, increased decay of NMDAR currents when treated with MK801, and increased minimal stimulation-elicited response, providing evidence that the Nrg3 mutation increases glutamate release probability. Notably, Nrg3 is a presynaptic protein that regulates the SNARE-complex assembly. Finally, increased Nrg3 levels, as observed in patients with severe mental illnesses, suppressed glutamatergic transmission. Together, these observations indicate that, unlike the prototype Nrg1, the effect of which is mediated by activating ErbB4 in interneurons, Nrg3 is critical in controlling glutamatergic transmission by regulating the SNARE complex at the presynaptic terminals, identifying a function of Nrg3 and revealing a pathophysiological mechanism for hypofunction of the glutamatergic pathway in Nrg3-related severe mental illnesses.

Mutations in Vps15 perturb neuronal migration in mice and are associated with neurodevelopmental disease in humans

The formation of the vertebrate brain requires the generation, migration, differentiation and survival of neurons. Genetic mutations that perturb these critical cellular events can result in malformations of the telencephalon, providing a molecular window into brain development. Here we report the identification of an N-ethyl-N-nitrosourea-induced mouse mutant characterized by a fractured hippocampal pyramidal cell layer, attributable to defects in neuronal migration. We show that this is caused by a hypomorphic mutation in Vps15 that perturbs endosomal-lysosomal trafficking and autophagy, resulting in an upregulation of Nischarin, which inhibits Pak1 signaling. The complete ablation of Vps15 results in the accumulation of autophagic substrates, the induction of apoptosis and severe cortical atrophy. Finally, we report that mutations in VPS15 are associated with cortical atrophy and epilepsy in humans. These data highlight the importance of the Vps15-Vps34 complex and the Nischarin-Pak1 signaling hub in the development of the telencephalon.

Lhx2 Expression in Postmitotic Cortical Neurons Initiates Assembly of the Thalamocortical Somatosensory Circuit

Cortical neurons must be specified and make the correct connections during development. Here, we examine a mechanism initiating neuronal circuit formation in the barrel cortex, a circuit comprising thalamocortical axons (TCAs) and layer 4 (L4) neurons. When Lhx2 is selectively deleted in postmitotic cortical neurons using conditional knockout (cKO) mice, L4 neurons in the barrel cortex are initially specified but fail to form cellular barrels or develop polarized dendrites. In Lhx2 cKO mice, TCAs from the thalamic ventral posterior nucleus reach the barrel cortex but fail to further arborize to form barrels. Several activity-regulated genes and genes involved in regulating barrel formation are downregulated in the Lhx2 cKO somatosensory cortex. Among them, Btbd3, an activity-regulated gene controlling dendritic development, is a direct downstream target of Lhx2. We find that Lhx2 confers neuronal competency for activity-dependent dendritic development in L4 neurons by inducing the expression of Btbd3.

Mn2+ dynamics in manganese-enhanced MRI (MEMRI): Cav1.2 channel-mediated uptake and preferential accumulation in projection terminals

Manganese-enhanced magnetic resonance imaging (MEMRI) exploits the biophysical similarity of Ca²⁺ and Mn²⁺ to map the brain's activity in vivo. However, to what extent different Ca²⁺ channels contribute to the enhanced signal that MEMRI provides and how Mn²⁺ dynamics influence Mn²⁺ brain accumulation after systemic administration of MnCl₂ are not yet fully understood. Here, we demonstrate that mice lacking the L-type Ca²⁺ channel 1.2 (Ca_v1.2) in the CNS show approximately 50% less increase in MEMRI contrast after repeated systemic MnCl₂ injections, as compared to control mice. In contrast, genetic deletion of L-type Ca²⁺ channel 1.3 (Ca_v1.3) did not reduce signal. Brain structure- or cell type-specific deletion of Ca_v1.2 in combination with voxel-wise MEMRI analysis revealed a preferential accumulation of Mn²⁺ in projection terminals, which was confirmed by local MnCl₂ administration to defined brain areas. Taken together, we provide unequivocal evidence that Ca_v1.2 represents an important channel for neuronal Mn²⁺ influx after systemic injections. We also show that after neuronal uptake, Mn²⁺ preferentially accumulates in projection terminals.

Neurodevelopmental disease-associated de novo mutations and rare sequence variants affect TRIO GDP/GTP exchange factor activity

Bipolar disorder, schizophrenia, autism and intellectual disability are complex neurodevelopmental disorders, debilitating millions of people. Therapeutic progress is limited by poor understanding of underlying molecular pathways. Using a targeted search, we identified an enrichment of de novo mutations in the gene encoding the 330-kDa triple functional domain (TRIO) protein associated with neurodevelopmental disorders. By generating multiple TRIO antibodies, we show that the smaller TRIO9 isoform is the major brain protein product, and its levels decrease after birth. TRIO9 contains two guanine nucleotide exchange factor (GEF) domains with distinct specificities: GEF1 activates both Rac1 and RhoG; GEF2 activates RhoA. To understand the impact of disease-associated de novo mutations and other rare sequence variants on TRIO function, we utilized two FRET-based biosensors: a Rac1 biosensor to study mutations in TRIO (T)GEF1, and a RhoA biosensor to study mutations in TGEF2. We discovered that one autism-associated de novo mutation in TGEF1 (K1431M), at the TGEF1/Rac1 interface, markedly decreased its overall activity toward Rac1. A schizophrenia-associated rare sequence variant in TGEF1 (F1538Intron) was substantially less active, normalized to protein level and expressed poorly. Overall, mutations in TGEF1 decreased GEF1 activity toward Rac1. One bipolar disorder-associated rare variant (M2145T) in TGEF2 impaired inhibition by the TGEF2 pleckstrin-homology domain, resulting in dramatically increased TGEF2 activity. Overall, genetic damage to both TGEF domains altered TRIO catalytic activity, decreasing TGEF1 activity and increasing TGEF2 activity. Importantly, both GEF changes are expected to decrease neurite outgrowth, perhaps consistent with their association with neurodevelopmental disorders.

Neuregulin 3 Mediates Cortical Plate Invasion and Laminar Allocation of GABAergic Interneurons

Neural circuits in the cerebral cortex consist of excitatory pyramidal cells and inhibitory interneurons. These two main classes of cortical neurons follow largely different genetic programs, yet they assemble into highly specialized circuits during development following a very precise choreography. Previous studies have shown that signals produced by pyramidal cells influence the migration of cortical interneurons, but the molecular nature of these factors has remained elusive. Here, we identified Neuregulin 3 (Nrg3) as a chemoattractive factor expressed by developing pyramidal cells that guides the allocation of cortical interneurons in the developing cortical plate. Gain- and loss-of-function approaches reveal that Nrg3 modulates the migration of interneurons into the cortical plate in a process that is dependent on the tyrosine kinase receptor ErbB4. Perturbation of Nrg3 signaling in conditional mutants leads to abnormal lamination of cortical interneurons. Nrg3 is therefore a critical mediator in the assembly of cortical inhibitory circuits.

•

Nrg3 acts a short-range chemoattractive molecule for cortical interneurons

•

Nrg3 functions through ErbB4 to attract interneurons into the cortical plate

•

Interneurons prefer Cxcl12 over Nrg3 during tangential migration

•

Disruption of Nrg3 signaling causes abnormal interneuron lamination in the cortex