Neocortex patterning by the secreted signaling molecule FGF8

The generalizability of cortical area parcellations across early childhood

The cerebral cortex consists of distinct areas that develop through intrinsic embryonic patterning and postnatal experiences. Accurate parcellation of these areas in neuroimaging studies improves statistical power and cross-study comparability. Given significant brain changes in volume, microstructure, and connectivity during early life, we hypothesized that cortical areas in 1- to 3-year-olds would differ markedly from neonates and increasingly resemble adult patterns as development progresses. Here, we parcellated the cerebral cortex into putative areas using local functional connectivity (FC) gradients in 92 toddlers at 2 years old. We demonstrate high reproducibility of these cortical areas across 1- to 3-year-olds in two independent datasets. The area boundaries in 1- to 3-year-olds were more similar to those in adults than those in neonates. While the age-specific group area parcellation better fits the underlying FC in individuals during the first 3 years, adult area parcellations still have utility in developmental studies, especially in children older than 6 years. Additionally, we provide connectivity-based community assignments of the area parcels, showing fragmented anterior and posterior components based on the strongest connectivity, yet alignment with adult systems when weaker connectivity was included.

Development of the rodent prefrontal cortex: circuit formation, plasticity, and impacts of early life stress

The prefrontal cortex (PFC), located at the anterior region of the cerebral cortex, is a multimodal association cortex essential for higher-order brain functions, including decision-making, attentional control, memory processing, and regulation of social behavior. Structural, circuit-level, and functional abnormalities in the PFC are often associated with neurodevelopmental disorders. Here, we review recent findings on the postnatal development of the PFC, with a particular emphasis on rodent studies, to elucidate how its structural and circuit properties are established during critical developmental windows and how these processes influence adult behaviors. Recent evidence also highlights the lasting effects of early life stress on the PFC structure, connectivity, and function. We explore potential mechanisms underlying these stress-induced alterations, with a focus on epigenetic regulation and its implications for PFC maturation and neurodevelopmental disorders. By integrating these insights, this review provides an overview of the developmental processes shaping the PFC and their implications for brain health and disease.

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.

The Principle of Cortical Development and Evolution

Human's robust cognitive abilities, including creativity and language, are made possible, at least in large part, by evolutionary changes made to the cerebral cortex. This paper reviews the biology and evolution of mammalian cortical radial glial cells (primary neural stem cells) and introduces the concept that a genetically step wise process, based on a core molecular pathway already in use, is the evolutionary process that has molded cortical neurogenesis. The core mechanism, which has been identified in our recent studies, is the extracellular signal-regulated kinase (ERK)-bone morphogenic protein 7 (BMP7)-GLI3 repressor form (GLI3R)-sonic hedgehog (SHH) positive feedback loop. Additionally, I propose that the molecular basis for cortical evolutionary dwarfism, exemplified by the lissencephalic mouse which originated from a larger gyrencephalic ancestor, is an increase in SHH signaling in radial glia, that antagonizes ERK-BMP7 signaling. Finally, I propose that: (1) SHH signaling is not a key regulator of primate cortical expansion and folding; (2) human cortical radial glial cells do not generate neocortical interneurons; (3) human-specific genes may not be essential for most cortical expansion. I hope this review assists colleagues in the field, guiding research to address gaps in our understanding of cortical development and evolution.

The Generalizability of Cortical Area Parcellations Across Early Childhood

The cerebral cortex consists of distinct areas that develop through intrinsic embryonic patterning and postnatal experiences. Accurate parcellation of these areas in neuroimaging studies improves statistical power and cross-study comparability. Given significant brain changes in volume, microstructure, and connectivity during early life, we hypothesized that cortical areas in 1- to 3-year-olds would differ markedly from neonates and increasingly resemble adult patterns as development progresses. Here, we parcellated the cerebral cortex into putative areas using local functional connectivity gradients in 92 toddlers at 2 years old. We demonstrate high reproducibility of these cortical regions across 1- to 3-year-olds in two independent datasets. The area boundaries in 1- to 3-year-olds were more similar to those in adults than those in neonates. While the age-specific group area parcellation better fit the underlying functional connectivity in individuals during the first 3 years, adult area parcellations might still have some utility in developmental studies, especially in children older than 6 years. Additionally, we provide connectivity-based community assignments of the parcels, showing fragmented anterior and posterior components based on the strongest connectivity, yet alignment with adult systems when weaker connectivity was included.

Generation of Prefrontal Cortex Neurons from Human Pluripotent Stem Cells Under Chemically Defined Conditions

Human pluripotent stem cells (hPSCs) have the potential to differentiate into all human somatic cell types allowing for an experimental platform to access otherwise inaccessible tissues through directed differentiation protocols. Access to tissue is especially critical for neurobiology where functional human tissue is rare. The prefrontal cortex is an evolutionarily expanded addition to the cerebral cortex, associated with higher order cognitive function. Here, we present a method to generate neural progenitors and post-mitotic neurons with a prefrontal cortical identity through the directed differentiation of hPSCs under defined conditions. Neural induction is achieved by inhibiting TGF beta and BMP signaling (termed dual-SMAD inhibition) and neural progenitors are subsequently patterned toward the prefrontal cortex lineage using recombinant FGF8. Cells generated using this protocol open possibilities to study the unique developmental and synaptic properties of the prefrontal cortex, as well as to understand its selective vulnerability in a number of human brain disorders.

The relationship between brain and visceral asymmetry: Evidence from situs inversus in humans

This review examines the relationship between visceral and brain asymmetry and explores whether their alignment observed in some vertebrate species also exists in humans. While the development of visceral and brain asymmetry may have occurred for different reasons, it is possible that the basic mechanisms for left-right differentiation of the visceral system were duplicated in the brain. We describe the main phenotypical anomalies and the general mechanism of left-right differentiation in vertebrates, followed by a systematic review of available human studies on behavioral and brain asymmetry in individuals with reversed visceral organization. The available evidence shows no direct link between human visceral and brain laterality. Most individuals with situs inversus totalis (SIT) show typical population biases for handedness and brain functional asymmetry, although an increased prevalence of atypical hemispheric segregation may be present. Perisylvian brain structural asymmetries also reveal the expected population bias in participants with SIT. However, several independent studies indicate that SIT is associated with a general reversal of the gross morphologic asymmetry of brain torque. Potential differences in brain structural and functional asymmetries between subtypes of situs inversus with ciliary and nonciliary causes remain to be determined.

Initial regional cytoarchitectonic differences in dorsal and orbitobasal human developing frontal cortex revealed by spatial transcriptomics

Early development of the human fetal cerebral cortex involves a set of precisely coordinated molecular processes that remains rather underexplored. Previous studies indicate that the laminar identity and the molecular specification of cortical neurons driven by genetic programming, as well as associated histogenetic events begin during early fetal development. Our recent study discovered unique regional cytoarchitectonic features in the developing human frontal lobe, including migratory waves of postmitotic neurons in the dorsal frontal cortex and the "double plate" feature in orbitobasal cortex (Kopić et al. in Cells 12:231, 2023). Notably, neurons of these two cytoarchitectonic features typically express deep projection neuron (DPN) markers (TBR1, TLE4, SOX5). This paper aims to conduct an in-depth investigation of these cytoarchitectonic features at the transcriptomic level, whilst preserving spatial information. Here, we employed NanoString GeoMx™ Digital Spatial Profiler (DSP) technology to examine gene expression differences in the transient cortical compartments of the dorsal and ventral regions of the developing frontal lobe, focusing specifically on 15 post-conceptional weeks (PCW), that is a critical period for subplate formation. We identified multiple differentially expressed genes between the transient cellular compartments of the dorsal and orbitobasal regions of the developing human frontal cortex. These new findings additionally confirm that regional patterning and specification of the prospective higher-order association prefrontal cortex emerges early in fetal development, contributing to the highly organized cortical architecture of the human brain.

Engineering regional diversity: A morphogen screen for patterned brain organoids

Morphogens orchestrate cellular diversity during nervous system development, yet a systematic approach to harnessing these signals in stem cell differentiation remains elusive. Amin et al.1 present a platform integrating parallel morphogen modulator screening with single-cell sequencing of neural organoids, reinforcing brain regionalization principles and enabling detailed cellular annotation.

From bedside to bench: New insights in epilepsy-associated tumors based on recent classification updates and animal models on brain tumor networks

Low-grade neuroepithelial tumors (LGNTs), particularly those with glioneuronal histology, are highly associated with pharmacoresistant epilepsy. Increasing research focused on these neoplastic lesions did not translate into drug discovery; and anticonvulsant or antitumor therapies are not available yet. During the last years, animal modeling has improved, thereby leading to the possibility of generating brain tumors in mice mimicking crucial genetic, molecular and immunohistological features. Among them, intraventricular in utero electroporation (IUE) has been proven to be a valuable tool for the generation of animal models for LGNTs allowing endogenous tumor growth within the mouse brain parenchyma. Epileptogenicity is mostly determined by the slow-growing patterns of these tumors, thus mirroring intrinsic interactions between tumor cells and surrounding neurons is crucial to investigate the mechanisms underlying convulsive activity. In this review, we provide an updated classification of the human LGNT and summarize the most recent data from human and animal models, with a focus on the crosstalk between brain tumors and neuronal function.

A simple method for gene expression in endo- and ectodermal cells in mouse embryos before neural tube closure

The lack of a widely accessible method for expressing genes of interest in wild-type embryos is a fundamental obstacle to understanding genetic regulation during embryonic development. In particular, only a few methods are available for introducing gene expression vectors into cells prior to neural tube closure, which is a period of drastic development for many tissues. In this study, we present a simple technique for injecting vectors into the amniotic cavity and allowing them to reach the ectodermal cells and the epithelia of endodermal organs of mouse embryos at E8.0 via in utero injection, using only a widely used optical fiber with an illuminator. Using this technique, retroviruses can be introduced to facilitate the labeling of cells in various tissues, including the brain, spinal cord, epidermis, and digestive and respiratory organs. We also demonstrated in utero electroporation of plasmid DNA into E7.0 and E8.0 embryos. Taking advantage of this method, we reveal the association between Ldb1 and the activity of the Neurog2 transcription factor in the mouse neocortex. This technique can aid in analyzing the roles of genes of interest during endo- and ectodermal development prior to neural tube closure.

FGF8-mediated gene regulation affects regional identity in human cerebral organoids

The morphogen FGF8 establishes graded positional cues imparting regional cellular responses via modulation of early target genes. The roles of FGF signaling and its effector genes remain poorly characterized in human experimental models mimicking early fetal telencephalic development. We used hiPSC-derived cerebral organoids as an in vitro platform to investigate the effect of FGF8 signaling on neural identity and differentiation. We found that FGF8 treatment increases cellular heterogeneity, leading to distinct telencephalic and mesencephalic-like domains that co-develop in multi-regional organoids. Within telencephalic regions, FGF8 affects the anteroposterior and dorsoventral identity of neural progenitors and the balance between GABAergic and glutamatergic neurons, thus impacting spontaneous neuronal network activity. Moreover, FGF8 efficiently modulates key regulators responsible for several human neurodevelopmental disorders. Overall, our results show that FGF8 signaling is directly involved in both regional patterning and cellular diversity in human cerebral organoids and in modulating genes associated with normal and pathological neural development.

BEAM: A combinatorial recombinase toolbox for binary gene expression and mosaic genetic analysis

We describe a binary expression aleatory mosaic (BEAM) system, which relies on DNA delivery by transfection or viral transduction along with nested recombinase activity to generate two genetically distinct, non-overlapping populations of cells for comparative analysis. Control cells labeled with red fluorescent protein (RFP) can be directly compared with experimental cells manipulated by genetic gain or loss of function and labeled with GFP. Importantly, BEAM incorporates recombinase-dependent signal amplification and delayed reporter expression to enable sharper delineation of control and experimental cells and to improve reliability relative to existing methods. We applied BEAM to a variety of known phenotypes to illustrate its advantages for identifying temporally or spatially aberrant phenotypes, for revealing changes in cell proliferation or death, and for controlling for procedural variability. In addition, we used BEAM to test the cortical protomap hypothesis at the individual radial unit level, revealing that area identity is cell autonomously specified in adjacent radial units.

ERK signaling expands mammalian cortical radial glial cells and extends the neurogenic period

The molecular basis for cortical expansion during evolution remains largely unknown. Here, we report that fibroblast growth factor (FGF)-extracellular signal-regulated kinase (ERK) signaling promotes the self-renewal and expansion of cortical radial glial (RG) cells. Furthermore, FGF-ERK signaling induces bone morphogenic protein 7 (Bmp7) expression in cortical RG cells, which increases the length of the neurogenic period. We demonstrate that ERK signaling and Sonic Hedgehog (SHH) signaling mutually inhibit each other in cortical RG cells. We provide evidence that ERK signaling is elevated in cortical RG cells during development and evolution. We propose that the expansion of the mammalian cortex, notably in human, is driven by the ERK-BMP7-GLI3R signaling pathway in cortical RG cells, which participates in a positive feedback loop through antagonizing SHH signaling. We also propose that the relatively short cortical neurogenic period in mice is partly due to mouse cortical RG cells receiving higher SHH signaling that antagonizes ERK signaling.

An enriched maternal environment and stereotypies of sows differentially affect the neuro-epigenome of brain regions related to emotionality in their piglets

Epigenetic mechanisms are important modulators of neurodevelopmental outcomes in the offspring of animals challenged during pregnancy. Pregnant sows living in a confined environment are challenged with stress and lack of stimulation which may result in the expression of stereotypies (repetitive behaviours without an apparent function). Little attention has been devoted to the postnatal effects of maternal stereotypies in the offspring. We investigated how the environment and stereotypies of pregnant sows affected the neuro-epigenome of their piglets. We focused on the amygdala, frontal cortex, and hippocampus, brain regions related to emotionality, learning, memory, and stress response. Differentially methylated regions (DMRs) were investigated in these brain regions of male piglets born from sows kept in an enriched vs a barren environment. Within the latter group of piglets, we compared the brain methylomes of piglets born from sows expressing stereotypies vs sows not expressing stereotypies. DMRs emerged in each comparison. While the epigenome of the hippocampus and frontal cortex of piglets is mainly affected by the maternal environment, the epigenome of the amygdala is mainly affected by maternal stereotypies. The molecular pathways and mechanisms triggered in the brains of piglets by maternal environment or stereotypies are different, which is reflected on the differential gene function associated to the DMRs found in each piglets' brain region . The present study is the first to investigate the neuro-epigenomic effects of maternal enrichment in pigs' offspring and the first to investigate the neuro-epigenomic effects of maternal stereotypies in the offspring of a mammal.

Molecular programs of regional specification and neural stem cell fate progression in macaque telencephalon

During early telencephalic development, intricate processes of regional patterning and neural stem cell (NSC) fate specification take place. However, our understanding of these processes in primates, including both conserved and species-specific features, remains limited. Here, we profiled 761,529 single-cell transcriptomes from multiple regions of the prenatal macaque telencephalon. We deciphered the molecular programs of the early organizing centers and their cross-talk with NSCs, revealing primate-biased galanin-like peptide (GALP) signaling in the anteroventral telencephalon. Regional transcriptomic variations were observed along the frontotemporal axis during early stages of neocortical NSC progression and in neurons and astrocytes. Additionally, we found that genes associated with neuropsychiatric disorders and brain cancer risk might play critical roles in the early telencephalic organizers and during NSC progression.

Golgi polarity shift instructs dendritic refinement in the neonatal cortex by mediating NMDA receptor signaling

Dendritic refinement is a critical component of activity-dependent neuronal circuit maturation, through which individual neurons establish specific connectivity with their target axons. Here, we demonstrate that the developmental shift of Golgi polarity is a key process in dendritic refinement. During neonatal development, the Golgi apparatus in layer 4 spiny stellate (SS) neurons in the mouse barrel cortex lose their original apical positioning and acquire laterally polarized distributions. This lateral Golgi polarity, which is oriented toward the barrel center, peaks on postnatal days 5-7 (P5-P7) and disappears by P15, which aligns with the developmental time course of SS neuron dendritic refinement. Genetic ablation of N-methyl-D-aspartate (NMDA) receptors, key players in dendritic refinement, disturbs the lateral Golgi polarity. Golgi polarity manipulation disrupts the asymmetric dendritic projection pattern and the primary-whisker-specific response of SS neurons. Our results elucidate activity-dependent Golgi dynamics and their critical role in developmental neuronal circuit refinement.

Experience-dependent structural plasticity of pyramidal neurons in the developing sensory cortices

Sensory experience regulates the structural and functional wiring of neuronal circuits, during development and throughout adulthood. Here, we review current knowledge of how experience affects structural plasticity of pyramidal neurons in the sensory cortices. We discuss the pros and cons of existing labeling approaches, as well as what structural parameters are most plastic. We further discuss how recent advances in sparse labeling of specific neuronal subtypes, as well as development of techniques that allow fast, high resolution imaging in large fields, would enable future studies to address currently unanswered questions in the field of structural plasticity.

Recent Genome-Editing Approaches toward Post-Implanted Fetuses in Mice

Genome editing, as exemplified by the CRISPR/Cas9 system, has recently been employed to effectively generate genetically modified animals and cells for the purpose of gene function analysis and disease model creation. There are at least four ways to induce genome editing in individuals: the first is to perform genome editing at the early preimplantation stage, such as fertilized eggs (zygotes), for the creation of whole genetically modified animals; the second is at post-implanted stages, as exemplified by the mid-gestational stages (E9 to E15), for targeting specific cell populations through in utero injection of viral vectors carrying genome-editing components or that of nonviral vectors carrying genome-editing components and subsequent in utero electroporation; the third is at the mid-gestational stages, as exemplified by tail-vein injection of genome-editing components into the pregnant females through which the genome-editing components can be transmitted to fetal cells via a placenta-blood barrier; and the last is at the newborn or adult stage, as exemplified by facial or tail-vein injection of genome-editing components. Here, we focus on the second and third approaches and will review the latest techniques for various methods concerning gene editing in developing fetuses.

In vivo two-photon calcium imaging of cortical neurons in neonatal mice

In vivo calcium imaging is essential to elucidate unique synchronous activities observed in the developing brain. Here, we present a protocol to image and analyze activity patterns in neonatal mouse neocortex in a single-cell level. We describe steps for in utero electroporation, cranial window surgery, two-photon imaging, and activity correlation analysis. This protocol facilitates the understanding of neuronal activities and activity-dependent circuit formation during development. For complete details on the use and execution of this protocol, please refer to Mizuno et al. (2014),¹ Mizuno et al. (2018a),² and Mizuno et al. (2018b).³.

Advances in in utero electroporation

In utero electroporation (IUE) is a technique developed in the early 2000s to transfect the neurons and neural progenitors of embryonic brains, thus enabling continued development in utero and subsequent analyses of neural development. Early IUE experiments focused on ectopic expression of plasmid DNA to analyze parameters such as neuron morphology and migration. Recent advances made in other fields, such as CRISPR/CAS9 genome editing, have been incorporated into IUE techniques as they were developed. Here, we provide a general review of the mechanics and techniques involved in IUE and explore the breadth of approaches that can be used in conjunction with IUE to study cortical development in a rodent model, with a focus on the novel advances in IUE techniques. We also highlight a few cases that exemplify the potential of IUE to study a broad range of questions in neural development.

Patterning the cerebral cortex into distinct functional domains during development

The cerebral cortex is compartmentalized into multiple regions, including the newly evolved neocortex and evolutionarily older paleocortex and archicortex. These broad cortical regions can be further subdivided into different functional domains, each with its own unique cytoarchitecture and distinct set of input and output projections to perform specific functions. While many excitatory projection neurons show region-specific gene expression profiles, the cells are derived from the seemingly uniform progenitors in the dorsal telencephalon. Much progress has been made in defining the genetic mechanisms involved in generating the morphological and functional diversity of the central nervous system. In this review, we summarize the current knowledge of mouse corticogenesis and discuss key events involved in cortical patterning during early developmental stages.

Laminar dynamics of deep projection neurons and mode of subplate formation are hallmarks of histogenetic subdivisions of the human cingulate cortex before onset of arealization

The cingulate gyrus, as a prominent part of the human limbic lobe, is involved in the integration and regulation of complex emotional, executive, motivational, and cognitive functions, attributed to several functional regions along the anteroposterior axis. In contrast to increasing knowledge of cingulate function in the adult brain, our knowledge of cingulate development is based primarily on classical neuroembryological studies. We aimed to reveal the laminar and cellular development of the various cingulate regions during the critical period from 7.5 to 15 postconceptional weeks (PCW) before the formation of Brodmann type arealization, employing diverse molecular markers on serial histological sections of postmortem human fetal brains. The study was performed by analysis of: (1) deep projection neuron (DPN) markers laminar dynamics, (2) all transient laminar compartments, and (3) characteristic subplate (SP) formation-expansion phase. We found that DPN markers labeling an incipient cortical plate (CP) were the first sign of regional differentiation of the dorsal isocortical and ventral mesocortical belt. Remarkably, increased width of the fibrillar marginal zone (MZ) towards the limbus, in parallel with the narrowing of CP containing DPN, as well as the diminishment of subventricular zone (SVZ) were reliable landmarks of early mesocortical differentiation. Finally, the SP formation pattern was shown to be a crucial event in the isocortical cingulate portion, given that the mesocortical belt is characterized by an incomplete CP delamination and absence of SP expansion. In conclusion, laminar DPN markers dynamics, together with the SVZ size and mode of SP formation indicate regional belt-like cingulate cortex differentiation before the corpus callosum expansion and several months before Brodmann type arealization.

Activity-dependent feedback regulation of thalamocortical axon development by Lhx2 in cortical layer 4 neurons

Establishing neuronal circuits requires interactions between pre- and postsynaptic neurons. While presynaptic neurons were shown to play instructive roles for the postsynaptic neurons, how postsynaptic neurons provide feedback to regulate the presynaptic neuronal development remains elusive. To elucidate the mechanisms for circuit formation, we study the development of barrel cortex (the primary sensory cortex, S1), whose development is instructed by presynaptic thalamocortical axons (TCAs). In the first postnatal weeks, TCA terminals arborize in layer (L) 4 to fill in the barrel center, but it is unclear how TCA development is regulated. Here, we reported that the deletion of Lhx2 specifically in the cortical neurons in the conditional knockout (cKO) leads to TCA arborization defects, which is accompanied with deficits in sensory-evoked and spontaneous cortical activities and impaired lesion-induced plasticity following early whisker follicle ablation. Reintroducing Lhx2 back in L4 neurons in cKO ameliorated TCA arborization and plasticity defects. By manipulating L4 neuronal activity, we further demonstrated that Lhx2 induces TCA arborization via an activity-dependent mechanism. Additionally, we identified the extracellular signaling protein Sema7a as an activity-dependent downstream target of Lhx2 in regulating TCA branching. Thus, we discovered a bottom-up feedback mechanism for the L4 neurons to regulate TCA development.

Single-Cell Labeling Strategies to Dissect Neuronal Structures and Local Functions

The brain network consists of ten billion neurons and is the most complex structure in the universe. Understanding the structure of complex brain networks and neuronal functions is one of the main goals of modern neuroscience. Since the seminal invention of Golgi staining, single-cell labeling methods have been among the most potent approaches for dissecting neuronal structures and neural circuits. Furthermore, the development of sparse single-cell transgenic methods has enabled single-cell gene knockout studies to examine the local functions of various genes in neural circuits and synapses. Here, we review non-transgenic single-cell labeling methods and recent advances in transgenic strategies for sparse single neuronal labeling. These methods and strategies will fundamentally contribute to the understanding of brain structure and function.

BEAM: a combinatorial recombinase toolbox for binary gene expression and mosaic analysis

Genetic mosaic analysis, in which mutant cells reside intermingled with wild-type cells, is a powerful experimental approach, but has not been widely used in mice because existing genome-based strategies require complicated and protracted breeding schemes. We have developed an alternative approach termed BEAM (for Binary Expression Aleatory Mosaic) that relies on sparse recombinase activation to generate two genetically distinct, non-overlapping populations of cells for comparative analysis. Following delivery of DNA constructs by transfection or viral transduction, combinatorial recombinase activity generates two distinct populations of cells labeled with either green or red fluorescent protein. Any gene of interest can be mis-expressed or deleted in one population for comparison with intermingled control cells. We have extensively optimized and characterized this system both in vitro and in vivo , and demonstrate its power for investigating cell autonomy, identifying temporally or spatially aberrant phenotypes, revealing changes in cell proliferation or death, and controlling for procedural variability.

Hemispheric Asymmetries of Individual Differences in Functional Connectivity

Resting-state fMRI studies have revealed that individuals exhibit stable, functionally meaningful divergences in large-scale network organization. The locations with strongest deviations (called network "variants") have a characteristic spatial distribution, with qualitative evidence from prior reports suggesting that this distribution differs across hemispheres. Hemispheric asymmetries can inform us on constraints guiding the development of these idiosyncratic regions. Here, we used data from the Human Connectome Project to systematically investigate hemispheric differences in network variants. Variants were significantly larger in the right hemisphere, particularly along the frontal operculum and medial frontal cortex. Variants in the left hemisphere appeared most commonly around the TPJ. We investigated how variant asymmetries vary by functional network and how they compare with typical network distributions. For some networks, variants seemingly increase group-average network asymmetries (e.g., the group-average language network is slightly bigger in the left hemisphere and variants also appeared more frequently in that hemisphere). For other networks, variants counter the group-average network asymmetries (e.g., the default mode network is slightly bigger in the left hemisphere, but variants were more frequent in the right hemisphere). Intriguingly, left- and right-handers differed in their network variant asymmetries for the cingulo-opercular and frontoparietal networks, suggesting that variant asymmetries are connected to lateralized traits. These findings demonstrate that idiosyncratic aspects of brain organization differ systematically across the hemispheres. We discuss how these asymmetries in brain organization may inform us on developmental constraints of network variants and how they may relate to functions differentially linked to the two hemispheres.

Early Regional Patterning in the Human Prefrontal Cortex Revealed by Laminar Dynamics of Deep Projection Neuron Markers

Early regional patterning and laminar position of cortical projection neurons is determined by activation and deactivation of transcriptional factors (TFs) and RNA binding proteins (RBPs) that regulate spatiotemporal framework of neurogenetic processes (proliferation, migration, aggregation, postmigratory differentiation, molecular identity acquisition, axonal growth, dendritic development, and synaptogenesis) within transient cellular compartments. Deep-layer projection neurons (DPN), subplate (SPN), and Cajal-Retzius neurons (CRN) are early-born cells involved in the establishment of basic laminar and regional cortical architecture; nonetheless, laminar dynamics of their molecular transcriptional markers remain underexplored. Here we aimed to analyze laminar dynamics of DPN markers, i.e., transcription factors TBR1, CTIP2, TLE4, SOX5, and RBP CELF1 on histological serial sections of the human frontal cortex between 7.5-15 postconceptional weeks (PCW) in reference to transient proliferative, migratory, and postmigratory compartments. The subtle signs of regional patterning were seen during the late preplate phase in the pattern of sublaminar organization of TBR1+/Reelin+ CRN and TBR1+ pioneering SPN. During the cortical plate (CP)-formation phase, TBR1+ neurons became radially aligned, forming continuity from a well-developed subventricular zone to CP showing clear lateral to medial regional gradients. The most prominent regional patterning was seen during the subplate formation phase (around 13 PCW) when a unique feature of the orbitobasal frontal cortex displays a "double plate" pattern. In other portions of the frontal cortex (lateral, dorsal, medial) deep portion of CP becomes loose and composed of TBR1+, CTIP2+, TLE4+, and CELF1+ neurons of layer six and later-born SPN, which later become constituents of the expanded SP (around 15 PCW). Overall, TFs and RBPs mark characteristic regional laminar dynamics of DPN, SPN, and CRN subpopulations during remarkably early fetal phases of the highly ordered association cortex development.

Generation of Homogeneous Populations of Cortical Interneurons from Human Pluripotent Stem Cells

Cortical interneurons (cINs), especially those that are derived from the medial ganglionic eminence (MGE) during early development, are associated with various neuropsychiatric disorders. Human pluripotent stem cell (hPSC)-derived cINs can provide unlimited cell sources for studying disease mechanisms and developing novel therapeutics. Here, we describe an optimized method to generate homogeneous cIN populations based on three-dimensional (3D) cIN sphere generation. This optimized differentiation system could sustain generated cINs relatively long term without compromising their survival or phenotypes.

Temporal and sequential transcriptional dynamics define lineage shifts in corticogenesis

The cerebral cortex contains billions of neurons, and their disorganization or misspecification leads to neurodevelopmental disorders. Understanding how the plethora of projection neuron subtypes are generated by cortical neural stem cells (NSCs) is a major challenge. Here, we focused on elucidating the transcriptional landscape of murine embryonic NSCs, basal progenitors (BPs), and newborn neurons (NBNs) throughout cortical development. We uncover dynamic shifts in transcriptional space over time and heterogeneity within each progenitor population. We identified signature hallmarks of NSC, BP, and NBN clusters and predict active transcriptional nodes and networks that contribute to neural fate specification. We find that the expression of receptors, ligands, and downstream pathway components is highly dynamic over time and throughout the lineage implying differential responsiveness to signals. Thus, we provide an expansive compendium of gene expression during cortical development that will be an invaluable resource for studying neural developmental processes and neurodevelopmental disorders.

Development of a 3D simulator for training the mouse in utero electroporation

In utero electroporation (IUE) requires high-level training in microinjection through the mouse uterine wall into the lateral ventricle of the mouse brain. Training for IUE is currently being performed in live mice as no artificial models allow simulations yet. This study aimed to develop an anatomically realistic 3D printed simulator to train IUE in mice. To this end, we created embryo models containing lateral ventricles. We coupled them to uterus models in six steps: (1) computed tomography imaging, (2) 3D model segmentation, (3) 3D model refinement, (4) mold creation to cast the actual model, (5) 3D mold printing, and (6) mold casting the molds with a mix of soft silicones to ensure the hardness and consistency of the uterus and embryo. The results showed that the simulator assembly successfully recreated the IUE. The compression test did not differ in the mechanical properties of the real embryo or in the required load for uterus displacement. Furthermore, more than 90% of the users approved the simulator as an introduction to IUE and considered that the simulator could help reduce the number of animals for training. Despite current limitations, our 3D simulator enabled a realistic experience for initial approximations to the IUE and is a real alternative for implementing the 3Rs. We are currently working on refining the model.

Comparing the development of cortex-wide gene expression patterns between two species in a common reference frame

Advances in sequencing techniques have made comparative studies of gene expression a current focus for understanding evolutionary and developmental processes. However, insights into the spatial expression of genes have been limited by a lack of robust methodology. To overcome this obstacle, we developed methods and software tools for quantifying and comparing tissue-wide spatial patterns of gene expression within and between species. Here, we compare cortex-wide expression of RZRβ and Id2 mRNA across early postnatal development in mice and voles. We show that patterns of RZRβ expression in neocortical layer 4 are highly conserved between species but develop rapidly in voles and much more gradually in mice, who show a marked expansion in the relative size of the putative primary visual area across the first postnatal week. Patterns of Id2 expression, by contrast, emerge in a dynamic and layer-specific sequence that is consistent between the two species. We suggest that these differences in the development of neocortical patterning reflect the independent evolution of brains, bodies, and sensory systems in the 35 million years since their last common ancestor.

Time and dose-dependent effects of FGF8-FGFR1 signaling in GnRH neurons derived from human pluripotent stem cells

Fibroblast growth factor 8 (FGF8), acting through the fibroblast growth factor receptor 1 (FGFR1), has an important role in the development of gonadotropin-releasing hormone-expressing neurons (GnRH neurons). We hypothesized that FGF8 regulates differentiation of human GnRH neurons in a time- and dose-dependent manner via FGFR1. To investigate this further, human pluripotent stem cells were differentiated during 10 days of dual-SMAD inhibition into neural progenitor cells, followed either by treatment with FGF8 at different concentrations (25 ng/ml, 50 ng/ml or 100 ng/ml) for 10 days or by treatment with 100 ng/ml FGF8 for different durations (2, 4, 6 or 10 days); cells were then matured through DAPT-induced inhibition of Notch signaling for 5 days into GnRH neurons. FGF8 induced expression of GNRH1 in a dose-dependent fashion and the duration of FGF8 exposure correlated positively with gene expression of GNRH1 (P<0.05, Rs=0.49). However, cells treated with 100 ng/ml FGF8 for 2 days induced the expression of genes, such as FOXG1, ETV5 and SPRY2, and continued FGF8 treatment induced the dynamic expression of several other genes. Moreover, during exposure to FGF8, FGFR1 localized to the cell surface and its specific inhibition with the FGFR1 inhibitor PD166866 reduced expression of GNRH1 (P<0.05). In neurons, FGFR1 also localized to the nucleus. Our results suggest that dose- and time-dependent FGF8 signaling via FGFR1 is indispensable for human GnRH neuron ontogeny.

This article has an associated First Person interview with the first author of the paper.

Summary: This article demonstrates the essential role FGF8–FGFR1 signaling has in the development of gonadotropin-releasing hormone (GnRH)-expressing neurons by using a human stem cell model.

Evaluation of advances in cortical development using model systems

Compared with that of even the closest primates, the human cortex displays a high degree of specialization and expansion that largely emerges developmentally. Although decades of research in the mouse and other model systems has revealed core tenets of cortical development that are well preserved across mammalian species, small deviations in transcription factor expression, novel cell types in primates and/or humans, and unique cortical architecture distinguish the human cortex. Importantly, many of the genes and signaling pathways thought to drive human-specific cortical expansion also leave the brain vulnerable to disease, as the misregulation of these factors is highly correlated with neurodevelopmental and neuropsychiatric disorders. However, creating a comprehensive understanding of human-specific cognition and disease remains challenging. Here, we review key stages of cortical development and highlight known or possible differences between model systems and the developing human brain. By identifying the developmental trajectories that may facilitate uniquely human traits, we highlight open questions in need of approaches to examine these processes in a human context and reveal translatable insights into human developmental disorders.

Thalamocortical axons regulate neurogenesis and laminar fates in the early sensory cortex

This study addresses how the cerebral cortex is partitioned into specialized areas during development. Although both early embryonic patterning and postnatal synaptic input from sensory thalamic nuclei are known to be critical, early roles of thalamic axons in area-specific regulation of cortical neurogenesis are poorly understood. We examined this by developing a genetic mouse model in which thalamocortical projections fail to properly form during embryogenesis, and found these axons are required not only for an enhanced production of superficial layer neurons but also for promoting the layer 4 cell fate, a hallmark of the primary sensory cortex. These findings provide a mechanism by which thalamocortical axons complement the intrinsic programs of neurogenesis and early fate specification.

Area-specific axonal projections from the mammalian thalamus shape unique cellular organization in target areas in the adult neocortex. How these axons control neurogenesis and early neuronal fate specification is poorly understood. By using mutant mice lacking the majority of thalamocortical axons, we show that these axons are required for the production and specification of the proper number of layer 4 neurons in primary sensory areas by the neonatal stage. Part of these area-specific roles is played by the thalamus-derived molecule, VGF. Our work reveals that extrinsic cues from sensory thalamic projections have an early role in the formation of cortical cytoarchitecture by enhancing the production and specification of layer 4 neurons.

In Utero Electroporation for Manipulation of Specific Neuronal Populations

The complexity of brain functions is supported by the heterogeneity of brain tissue and millisecond-scale information processing. Understanding how complex neural circuits control animal behavior requires the precise manipulation of specific neuronal subtypes at high spatiotemporal resolution. In utero electroporation, when combined with optogenetics, is a powerful method for precisely controlling the activity of specific neurons. Optogenetics allows for the control of cellular membrane potentials through light-sensitive ion channels artificially expressed in the plasma membrane of neurons. Here, we first review the basic mechanisms and characteristics of in utero electroporation. Then, we discuss recent applications of in utero electroporation combined with optogenetics to investigate the functions and characteristics of specific regions, layers, and cell types. These techniques will pave the way for further advances in understanding the complex neuronal and circuit mechanisms that underlie behavioral outputs.

Genetic Regulation of Vertebrate Forebrain Development by Homeobox Genes

Forebrain development in vertebrates is regulated by transcription factors encoded by homeobox, bHLH and forkhead gene families throughout the progressive and overlapping stages of neural induction and patterning, regional specification and generation of neurons and glia from central nervous system (CNS) progenitor cells. Moreover, cell fate decisions, differentiation and migration of these committed CNS progenitors are controlled by the gene regulatory networks that are regulated by various homeodomain-containing transcription factors, including but not limited to those of the Pax (paired), Nkx, Otx (orthodenticle), Gsx/Gsh (genetic screened), and Dlx (distal-less) homeobox gene families. This comprehensive review outlines the integral role of key homeobox transcription factors and their target genes on forebrain development, focused primarily on the telencephalon. Furthermore, links of these transcription factors to human diseases, such as neurodevelopmental disorders and brain tumors are provided.

Visualizing Cortical Development and Evolution: A Toolkit Update

Visualizing the process of neural circuit formation during neurogenesis, using genetically modified animals or somatic transgenesis of exogenous plasmids, has become a key to decipher cortical development and evolution. In contrast to the establishment of transgenic animals, the designing and preparation of genes of interest into plasmids are simple and easy, dispensing with time-consuming germline modifications. These advantages have led to neuron labeling based on somatic transgenesis. In particular, mammalian expression plasmid, CRISPR-Cas9, and DNA transposon systems, have become widely used for neuronal visualization and functional analysis related to lineage labeling during cortical development. In this review, we discuss the advantages and limitations of these recently developed techniques.

Thalamocortical axons control the cytoarchitecture of neocortical layers by area-specific supply of VGF

Neuronal abundance and thickness of each cortical layer are specific to each area, but how this fundamental feature arises during development remains poorly understood. While some of area-specific features are controlled by intrinsic cues such as morphogens and transcription factors, the exact influence and mechanisms of action by cues extrinsic to the cortex, in particular the thalamic axons, have not been fully established. Here, we identify a thalamus-derived factor, VGF, which is indispensable for thalamocortical axons to maintain the proper amount of layer 4 neurons in the mouse sensory cortices. This process is prerequisite for further maturation of the primary somatosensory area, such as barrel field formation instructed by a neuronal activity-dependent mechanism. Our results provide an actual case in which highly site-specific axon projection confers further regional complexity upon the target field through locally secreting signaling molecules from axon terminals.

Dual in Utero Electroporation in Mice to Manipulate Two Specific Neuronal Populations in the Developing Cortex

Precise regulation of gene expression during development in cortical neurons is essential for the establishment and maintenance of neuronal connectivity and higher-order cognition. Dual in utero electroporation provides a precise and effective tool to label and manipulate gene expression in multiple neuronal populations within a circuit in a spatially and temporally regulated manner. In addition, this technique allows for morphophysiological investigations into neuronal development and connectivity following cell-specific gene manipulations. Here, we detail the dual in utero electroporation protocol.

Structural and Functional Aspects of the Neurodevelopmental Gene NR2F1: From Animal Models to Human Pathology

The assembly and maturation of the mammalian brain result from an intricate cascade of highly coordinated developmental events, such as cell proliferation, migration, and differentiation. Any impairment of this delicate multi-factorial process can lead to complex neurodevelopmental diseases, sharing common pathogenic mechanisms and molecular pathways resulting in multiple clinical signs. A recently described monogenic neurodevelopmental syndrome named Bosch-Boonstra-Schaaf Optic Atrophy Syndrome (BBSOAS) is caused by NR2F1 haploinsufficiency. The NR2F1 gene, coding for a transcriptional regulator belonging to the steroid/thyroid hormone receptor superfamily, is known to play key roles in several brain developmental processes, from proliferation and differentiation of neural progenitors to migration and identity acquisition of neocortical neurons. In a clinical context, the disruption of these cellular processes could underlie the pathogenesis of several symptoms affecting BBSOAS patients, such as intellectual disability, visual impairment, epilepsy, and autistic traits. In this review, we will introduce NR2F1 protein structure, molecular functioning, and expression profile in the developing mouse brain. Then, we will focus on Nr2f1 several functions during cortical development, from neocortical area and cell-type specification to maturation of network activity, hippocampal development governing learning behaviors, assembly of the visual system, and finally establishment of cortico-spinal descending tracts regulating motor execution. Whenever possible, we will link experimental findings in animal or cellular models to corresponding features of the human pathology. Finally, we will highlight some of the unresolved questions on the diverse functions played by Nr2f1 during brain development, in order to propose future research directions. All in all, we believe that understanding BBSOAS mechanisms will contribute to further unveiling pathophysiological mechanisms shared by several neurodevelopmental disorders and eventually lead to effective treatments.

Regulation of prefrontal patterning and connectivity by retinoic acid

The prefrontal cortex (PFC) and its connections with the mediodorsal thalamus are crucial for cognitive flexibility and working memory1 and are thought to be altered in disorders such as autism2,3 and schizophrenia4,5. Although developmental mechanisms that govern the regional patterning of the cerebral cortex have been characterized in rodents6-9, the mechanisms that underlie the development of PFC-mediodorsal thalamus connectivity and the lateral expansion of the PFC with a distinct granular layer 4 in primates10,11 remain unknown. Here we report an anterior (frontal) to posterior (temporal), PFC-enriched gradient of retinoic acid, a signalling molecule that regulates neural development and function12-15, and we identify genes that are regulated by retinoic acid in the neocortex of humans and macaques at the early and middle stages of fetal development. We observed several potential sources of retinoic acid, including the expression and cortical expansion of retinoic-acid-synthesizing enzymes specifically in primates as compared to mice. Furthermore, retinoic acid signalling is largely confined to the prospective PFC by CYP26B1, a retinoic-acid-catabolizing enzyme, which is upregulated in the prospective motor cortex. Genetic deletions in mice revealed that retinoic acid signalling through the retinoic acid receptors RXRG and RARB, as well as CYP26B1-dependent catabolism, are involved in proper molecular patterning of prefrontal and motor areas, development of PFC-mediodorsal thalamus connectivity, intra-PFC dendritic spinogenesis and expression of the layer 4 marker RORB. Together, these findings show that retinoic acid signalling has a critical role in the development of the PFC and, potentially, in its evolutionary expansion.

Cerebellar Differentiation from Human Stem Cells Through Retinoid, Wnt, and Sonic Hedgehog Pathways

Differentiating cerebellar organoids can be challenging due to complex cell organization and structure in the cerebellum. Different approaches were investigated to recapitulate differentiation process of the cerebellum from human-induced pluripotent stem cells (hiPSCs) without high efficiency. This study was carried out to test the hypothesis that the combination of different signaling factors including retinoic acid (RA), Wnt activator, and sonic hedgehog (SHH) activator promotes the cerebellar differentiation of hiPSCs. Wnt, RA, and SHH pathways were activated by CHIR99021 (CHIR), RA, and purmorphamine (PMR), respectively. Different combinations of the morphogens (RA/CHIR, RA/PMR, CHIR/PMR, and RA/CHIR/PMR) were utilized, and the spheroids (day 35) were characterized for the markers of three cerebellum layers (the molecular layer, the Purkinje cell layer, and the granule cell layer). Of all the combinations tested, RA/CHIR/PMR promoted both the Purkinje cell layer and the granule cell layer differentiation. The cells also exhibited electrophysiological characteristics using whole-cell patch clamp recording, especially demonstrating Purkinje cell electrophysiology. This study should advance the understanding of different signaling pathways during cerebellar development to engineer cerebellum organoids for drug screening and disease modeling. Impact statement This study investigated the synergistic effects of retinoic acid, Wnt activator, and sonic hedgehog activator on cerebellar patterning of human-induced pluripotent stem cell (hiPSC) spheroids and organoids. The results indicate that the combination promotes the differentiation of the Purkinje cell layer and the granule cell layer. The cells also exhibit electrophysiological characteristics using whole-cell patch clamp recording, especially demonstrating Purkinje cell electrophysiology. The findings are significant for understanding the biochemical signaling of three-dimensional microenvironment on neural patterning of hiPSCs for applications in organoid engineering, disease modeling, and drug screening.

Combining electrophysiology and optogenetics for functional screening of pyramidal neurons in the mouse prefrontal cortex

Here, we present a comprehensive protocol to analyze the roles of disease-related genes in synaptic transmission. We have developed a pipeline of electrophysiological techniques and combined these with optogenetics in the medial prefrontal cortex of mice. This methodology provides a cost-effective, faster, and easier screening approach to elucidate functional aspects of single genes in several regions in the mouse brain such as a specific layer of the mPFC. For complete details on the use and execution of this protocol, please refer to Nagahama et al. (2020) and Sacai et al. (2020).

Variation of Human Neural Stem Cells Generating Organizer States In Vitro before Committing to Cortical Excitatory or Inhibitory Neuronal Fates

Better understanding of the progression of neural stem cells (NSCs) in the developing cerebral cortex is important for modeling neurogenesis and defining the pathogenesis of neuropsychiatric disorders. Here, we use RNA sequencing, cell imaging, and lineage tracing of mouse and human in vitro NSCs and monkey brain sections to model the generation of cortical neuronal fates. We show that conserved signaling mechanisms regulate the acute transition from proliferative NSCs to committed glutamatergic excitatory neurons. As human telencephalic NSCs develop from pluripotency in vitro, they transition through organizer states that spatially pattern the cortex before generating glutamatergic precursor fates. NSCs derived from multiple human pluripotent lines vary in these early patterning states, leading differentially to dorsal or ventral telencephalic fates. This work furthers systematic analyses of the earliest patterning events that generate the major neuronal trajectories of the human telencephalon.

Micali et al. report that human telencephalic NSCs in vitro transition through the organizer states that pattern the neocortex. Human pluripotent lines vary in organizer formation, generating divergent neuronal differentiation trajectories biased toward dorsal or ventral telencephalic fates and opening further analysis of the earliest cortical specification events.

A Multiplex Human Pluripotent Stem Cell Platform Defines Molecular and Functional Subclasses of Autism-Related Genes

Autism is a clinically heterogeneous neurodevelopmental disorder characterized by impaired social interactions, restricted interests, and repetitive behaviors. Despite significant advances in the genetics of autism, understanding how genetic changes perturb brain development and affect clinical symptoms remains elusive. Here, we present a multiplex human pluripotent stem cell (hPSC) platform, in which 30 isogenic disease lines are pooled in a single dish and differentiated into prefrontal cortex (PFC) lineages to efficiently test early-developmental hypotheses of autism. We define subgroups of autism mutations that perturb PFC neurogenesis and are correlated to abnormal WNT/βcatenin responses. Class 1 mutations (8 of 27) inhibit while class 2 mutations (5 of 27) enhance PFC neurogenesis. Remarkably, autism patient data reveal that individuals carrying subclass-specific mutations differ clinically in their corresponding language acquisition profiles. Our study provides a framework to disentangle genetic heterogeneity associated with autism and points toward converging molecular and developmental pathways of diverse autism-associated mutations.

Atypical Brain Asymmetry in Human Situs Inversus: Gut Feeling or Real Evidence?

The alignment of visceral and brain asymmetry observed in some vertebrate species raises the question of whether this association also exists in humans. While the visceral and brain systems may have developed asymmetry for different reasons, basic visceral left–right differentiation mechanisms could have been duplicated to establish brain asymmetry. We describe the main phenotypical anomalies and the general mechanism of left–right differentiation of vertebrate visceral and brain laterality. Next, we systematically review the available human studies that explored the prevalence of atypical behavioral and brain asymmetry in visceral situs anomalies, which almost exclusively involved participants with the mirrored visceral organization (situs inversus). The data show no direct link between human visceral and brain functional laterality as most participants with situs inversus show the typical population bias for handedness and brain functional asymmetry, although an increased prevalence of functional crowding may be present. At the same time, several independent studies present evidence for a possible relation between situs inversus and the gross morphological asymmetry of the brain torque with potential differences between subtypes of situs inversus with ciliary and non-ciliary etiologies.

A role for sustained MAPK activity in the mouse ventral telencephalon

The MAPK pathway is a major growth signal that has been implicated during the development of progenitors, neurons, and glia in the embryonic brain. Here, we show that the MAPK pathway plays an important role in the generation of distinct cell types from progenitors in the ventral telencephalon. Our data reveal that phospho-p44/42 (called p-ERK1/2) and the ETS transcription factor Etv5, both downstream effectors in the MAPK pathway, show a regional bias in expression during ventral telencephalic development, with enriched expression in the dorsal region of the LGE and ventral region of the MGE at E13.5 and E15.5. Interestingly, expression of both factors becomes more uniform in ventricular zone (VZ) progenitors by E18.5. To gain insight into the role of MAPK activity during progenitor cell development, we used a cre inducible constitutively active MEK1 allele (Rosa^MEK1DD/+) in combination with a ventral telencephalon enriched cre (Gsx2e-cre) or a dorsal telencephalon enriched cre (Emx1^cre/+). Sustained MEK/MAPK activity in the ventral telencephalon (Gsx2e-cre; Rosa^MEK1DD/+) expanded dorsal lateral ganglionic eminence (dLGE) enriched genes (Gsx2 and Sp8) and oligodendrocyte progenitor cell (OPC) markers (Olig2, Pdgfrα, and Sox10), and also reduced markers in the ventral (v) LGE domain (Isl1 and Foxp1). Activation of MEK/MAPK activity in the dorsal telencephalon (Emx1^cre/+; Rosa^MEK1DD/+) did not initially activate the expression of dLGE or OPC genes at E15.5 but ectopic expression of Gsx2 and OPC markers were observed at E18.5. These results support the idea that MAPK activity as readout by p-ERK1/2 and Etv5 expression is enriched in distinct subdomains of ventral telencephalic progenitors during development. In addition, sustained activation of the MEK/MAPK pathway in the ventral or dorsal telencephalon influences dLGE and OPC identity from progenitors.

Generation of extracellular morphogen gradients: the case for diffusion

Cells within developing tissues rely on morphogens to assess positional information. Passive diffusion is the most parsimonious transport model for long-range morphogen gradient formation but does not, on its own, readily explain scaling, robustness and planar transport. Here, we argue that diffusion is sufficient to ensure robust morphogen gradient formation in a variety of tissues if the interactions between morphogens and their extracellular binders are considered. A current challenge is to assess how the affinity for extracellular binders, as well as other biophysical and cell biological parameters, determines gradient dynamics and shape in a diffusion-based transport system. Technological advances in genome editing, tissue engineering, live imaging and in vivo biophysics are now facilitating measurement of these parameters, paving the way for mathematical modelling and a quantitative understanding of morphogen gradient formation and modulation.