Regulation of Cerebral Cortex Folding by Controlling Neuronal Migration via FLRT Adhesion Molecules

Mechanical Forces Orchestrate Brain Development

During brain development, progenitors generate successive waves of neurons that populate distinct cerebral regions, where they settle and differentiate within layers or nuclei. While migrating and differentiating, neurons are subjected to mechanical forces arising from the extracellular matrix, and their interaction with neighboring cells. Changes in brain biomechanical properties, during its formation or aging, are converted in neural cells by mechanotransduction into intracellular signals that control key neurobiological processes. Here, we summarize recent findings that support the contribution of mechanobiology to neurodevelopment, with focus on the cerebral cortex. Also discussed are the existing toolbox and emerging technologies made available to assess and manipulate the physical properties of neurons and their environment.

Toward a neurospheroid niche model: optimizing embedded 3D bioprinting for fabrication of neurospheroid brain-like co-culture constructs

A crucial step in creating reliablein vitroplatforms for neural development and disorder studies is the reproduction of the multicellular three-dimensional (3D) brain microenvironment and the capturing of cell-cell interactions within the model. The power of self-organization of diverse cell types into brain spheroids could be harnessed to study mechanisms underlying brain development trajectory and diseases. A challenge of current 3D organoid and spheroid models grown in petri-dishes is the lack of control over cellular localization and diversity. To overcome this limitation, neural spheroids can be patterned into customizable 3D structures using microfabrication. We developed a 3D brain-like co-culture construct using embedded 3D bioprinting as a flexible solution for composing heterogenous neural populations with neurospheroids and glia. Specifically, neurospheroid-laden free-standing 3D structures were fabricated in an engineered astrocyte-laden support bath resembling a neural stem cell niche environment. A photo-crosslinkable bioink and a thermal-healing supporting bath were engineered to mimic the mechanical modulus of soft tissue while supporting the formation of self-organizing neurospheroids within elaborate 3D networks. Moreover, bioprinted neurospheroid-laden structures exhibited the capability to differentiate into neuronal cells. These brain-like co-cultures could provide a reproducible platform for modeling neurological diseases, neural regeneration, and drug development and repurposing.

Neuronal migration and disorders - an update

This review highlights genes, proteins and subcellular mechanisms, recently shown to influence cortical neuronal migration. A current view on mechanisms which become disrupted in a diverse array of migration disorders is presented. The microtubule (MT) cytoskeleton is a major player in migrating neurons. Recently, variable impacts on MTs have been revealed in different cell compartments. Thus there are a multiplicity of effects involving centrosomal, microtubule-associated, as well as motor proteins. However, other causative factors also emerge, illuminating cortical neuronal migration research. These include disruptions of the actin cytoskeleton, the extracellular matrix, different adhesion molecules and signaling pathways, especially revealed in disorders such as periventricular heterotopia. These recent advances often involve the use of human in vitro models as well as model organisms. Focusing on cell-type specific knockouts and knockins, as well as generating omics and functional data, all seem critical for an integrated view on neuronal migration dysfunction.

The Subplate: A Potential Driver of Cortical Folding?

In many species of Mammalia, the surface of the brain develops from a smooth structure to one with many fissures and folds, allowing for vast expansion of the surface area of the cortex. The importance of understanding what drives cortical folding extends beyond mere curiosity, as conditions such as preterm birth, intrauterine growth restriction, and fetal alcohol syndrome are associated with impaired folding in the infant and child. Despite being a key feature of brain development, the mechanisms driving cortical folding remain largely unknown. In this review we discuss the possible role of the subplate, a developmentally transient compartment, in directing region-dependent development leading to sulcal and gyral formation. We discuss the development of the subplate in species with lissencephalic and gyrencephalic cortices, the characteristics of the cells found in the subplate, and the possible presence of molecular cues that guide axons into, and out of, the overlying and multilayered cortex before the appearance of definitive cortical folds. An understanding of what drives cortical folding is likely to help in understanding the origins of abnormal folding patterns in clinical pathologies.

Non-Cell-Autonomous Mechanisms in Radial Projection Neuron Migration in the Developing Cerebral Cortex

Concerted radial migration of newly born cortical projection neurons, from their birthplace to their final target lamina, is a key step in the assembly of the cerebral cortex. The cellular and molecular mechanisms regulating the specific sequential steps of radial neuronal migration in vivo are however still unclear, let alone the effects and interactions with the extracellular environment. In any in vivo context, cells will always be exposed to a complex extracellular environment consisting of (1) secreted factors acting as potential signaling cues, (2) the extracellular matrix, and (3) other cells providing cell–cell interaction through receptors and/or direct physical stimuli. Most studies so far have described and focused mainly on intrinsic cell-autonomous gene functions in neuronal migration but there is accumulating evidence that non-cell-autonomous-, local-, systemic-, and/or whole tissue-wide effects substantially contribute to the regulation of radial neuronal migration. These non-cell-autonomous effects may differentially affect cortical neuron migration in distinct cellular environments. However, the cellular and molecular natures of such non-cell-autonomous mechanisms are mostly unknown. Furthermore, physical forces due to collective migration and/or community effects (i.e., interactions with surrounding cells) may play important roles in neocortical projection neuron migration. In this concise review, we first outline distinct models of non-cell-autonomous interactions of cortical projection neurons along their radial migration trajectory during development. We then summarize experimental assays and platforms that can be utilized to visualize and potentially probe non-cell-autonomous mechanisms. Lastly, we define key questions to address in the future.

How Do Electric Fields Coordinate Neuronal Migration and Maturation in the Developing Cortex?

During development the vast majority of cells that will later compose the mature cerebral cortex undergo extensive migration to reach their final position. In addition to intrinsically distinct migratory behaviors, cells encounter and respond to vastly different microenvironments. These range from axonal tracts to cell-dense matrices, electrically active regions and extracellular matrix components, which may all change overtime. Furthermore, migrating neurons themselves not only adapt to their microenvironment but also modify the local niche through cell-cell contacts, secreted factors and ions. In the radial dimension, the developing cortex is roughly divided into dense progenitor and cortical plate territories, and a less crowded intermediate zone. The cortical plate is bordered by the subplate and the marginal zone, which are populated by neurons with high electrical activity and characterized by sophisticated neuritic ramifications. Neuronal migration is influenced by these boundaries resulting in dramatic changes in migratory behaviors as well as morphology and electrical activity. Modifications in the levels of any of these parameters can lead to alterations and even arrest of migration. Recent work indicates that morphology and electrical activity of migrating neuron are interconnected and the aim of this review is to explore the extent of this connection. We will discuss on one hand how the response of migrating neurons is altered upon modification of their intrinsic electrical properties and whether, on the other hand, the electrical properties of the cellular environment can modify the morphology and electrical activity of migrating cortical neurons.

FLRTing Neurons in Cortical Migration During Cerebral Cortex Development

During development, two coordinated events shape the morphology of the mammalian cerebral cortex, leading to the cortex's columnar and layered structure: the proliferation of neuronal progenitors and cortical migration. Pyramidal neurons originating from germinal zones migrate along radial glial fibers to their final position in the cortical plate by both radial migration and tangential dispersion. These processes rely on the delicate balance of intercellular adhesive and repulsive signaling that takes place between neurons interacting with different substrates and guidance cues. Here, we focus on the function of the cell adhesion molecules fibronectin leucine-rich repeat transmembrane proteins (FLRTs) in regulating both the radial migration of neurons, as well as their tangential spread, and the impact these processes have on cortex morphogenesis. In combining structural and functional analysis, recent studies have begun to reveal how FLRT-mediated responses are precisely tuned - from forming different protein complexes to modulate either cell adhesion or repulsion in neurons. These approaches provide a deeper understanding of the context-dependent interactions of FLRTs with multiple receptors involved in axon guidance and synapse formation that contribute to finely regulated neuronal migration.

NR2F1 regulates regional progenitor dynamics in the mouse neocortex and cortical gyrification in BBSOAS patients

The relationships between impaired cortical development and consequent malformations in neurodevelopmental disorders, as well as the genes implicated in these processes, are not fully elucidated to date. In this study, we report six novel cases of patients affected by BBSOAS (Boonstra‐Bosch‐Schaff optic atrophy syndrome), a newly emerging rare neurodevelopmental disorder, caused by loss‐of‐function mutations of the transcriptional regulator NR2F1. Young patients with NR2F1 haploinsufficiency display mild to moderate intellectual disability and show reproducible polymicrogyria‐like brain malformations in the parietal and occipital cortex. Using a recently established BBSOAS mouse model, we found that Nr2f1 regionally controls long‐term self‐renewal of neural progenitor cells via modulation of cell cycle genes and key cortical development master genes, such as Pax6. In the human fetal cortex, distinct NR2F1 expression levels encompass gyri and sulci and correlate with local degrees of neurogenic activity. In addition, reduced NR2F1 levels in cerebral organoids affect neurogenesis and PAX6 expression. We propose NR2F1 as an area‐specific regulator of mouse and human brain morphology and a novel causative gene of abnormal gyrification.

Optimizing Nervous System-Specific Gene Targeting with Cre Driver Lines: Prevalence of Germline Recombination and Influencing Factors

The Cre-loxP system is invaluable for spatial and temporal control of gene knockout, knockin, and reporter expression in the mouse nervous system. However, we report varying probabilities of unexpected germline recombination in distinct Cre driver lines designed for nervous system-specific recombination. Selective maternal or paternal germline recombination is showcased with sample Cre lines. Collated data reveal germline recombination in over half of 64 commonly used Cre driver lines, in most cases with a parental sex bias related to Cre expression in sperm or oocytes. Slight differences among Cre driver lines utilizing common transcriptional control elements affect germline recombination rates. Specific target loci demonstrated differential recombination; thus, reporters are not reliable proxies for another locus of interest. Similar principles apply to other recombinase systems and other genetically targeted organisms. We hereby draw attention to the prevalence of germline recombination and provide guidelines to inform future research for the neuroscience and broader molecular genetics communities.

ECE2 regulates neurogenesis and neuronal migration during human cortical development

During embryonic development, excitatory projection neurons migrate in the cerebral cortex giving rise to organised layers. Periventricular heterotopia (PH) is a group of aetiologically heterogeneous disorders in which a subpopulation of newborn projection neurons fails to initiate their radial migration to the cortex, ultimately resulting in bands or nodules of grey matter lining the lateral ventricles. Although a number of genes have been implicated in its cause, currently they only satisfactorily explain the pathogenesis of the condition for 50% of patients. Novel gene discovery is complicated by the extreme genetic heterogeneity recently described to underlie its cause. Here, we study the neurodevelopmental role of endothelin‐converting enzyme‐2 (ECE2) for which two biallelic variants have been identified in two separate patients with PH. Our results show that manipulation of ECE2 levels in human cerebral organoids and in the developing mouse cortex leads to ectopic localisation of neural progenitors and neurons. We uncover the role of ECE2 in neurogenesis, and mechanistically, we identify its involvement in the generation and secretion of extracellular matrix proteins in addition to cytoskeleton and adhesion.

Neuronal strategies for meeting the right partner during brain wiring

Two neurons can only form a synapse if their axonal and dendritic projections meet at the same time and place. While spatiotemporal proximity is necessary for synapse formation, it remains unclear to what extent the underlying positional strategies are sufficient to ensure synapse formation between the right partners. Many neurons readily form synapses with wrong partners if they find themselves at the wrong place or time. Minimally, restricting spatiotemporal proximity can prevent incorrect synapses. Maximally, restricting encounters in time and space could be sufficient to ensure correct partnerships between neurons that can form synapses promiscuously. In this review we explore recent findings on positional strategies during developmental growth that contribute to precise outcomes in brain wiring.

Multiple rare inherited variants in a four generation schizophrenia family offer leads for complex mode of disease inheritance

Schizophrenia is a clinically and genetically heterogeneous neuropsychiatric disorder, with a polygenic basis but identification of the specific determinants is a continuing challenge. In this study, we analyzed a multigenerational family, with all healthy individuals in the first two generations, and four progeny affected with schizophrenia in the subsequent two generations, using whole exome sequencing. We identified five rare protein sequence altering heterozygous variants, in five different genes namely SMARCA5, PDE1B, TNIK, SMARCA2 and FLRT shared among all affected members and predicted to be damaging. Variants in SMARCA5 and PDE1B were inherited from the unaffected father whereas variants in TNIK, SMARCA2 and FLRT1 were inherited from the unaffected mother in all the three affected individuals in the third generation; and notably all these five variants were transmitted by an affected mother to her affected son. Microsatellite based analysis lent a modest linkage support (LOD score of 1.2; θ=0.0 at each variant). Of note, analysis of exome data of an ancestry matched unrelated schizophrenia cohort (n = 350), revealed a total of 16 rare variants (MAF < 0.01) in these five genes. Interestingly, these five genes involved in neurodevelopmental and/or neurotransmitter signaling processes are implicated in the etiology of schizophrenia previously. This study provides good evidence for a likely cumulative contribution of multiple rare variants from disease relevant genes with a threshold effect in disease development and seems to explain the unusual disease transmission pattern generally witnessed in such conditions, but warrants extensive replication efforts in families with similar complex disease inheritance profiles.

Structural Basis of Teneurin-Latrophilin Interaction in Repulsive Guidance of Migrating Neurons

Teneurins are ancient metazoan cell adhesion receptors that control brain development and neuronal wiring in higher animals. The extracellular C terminus binds the adhesion GPCR Latrophilin, forming a trans-cellular complex with synaptogenic functions. However, Teneurins, Latrophilins, and FLRT proteins are also expressed during murine cortical cell migration at earlier developmental stages. Here, we present crystal structures of Teneurin-Latrophilin complexes that reveal how the lectin and olfactomedin domains of Latrophilin bind across a spiraling beta-barrel domain of Teneurin, the YD shell. We couple structure-based protein engineering to biophysical analysis, cell migration assays, and in utero electroporation experiments to probe the importance of the interaction in cortical neuron migration. We show that binding of Latrophilins to Teneurins and FLRTs directs the migration of neurons using a contact repulsion-dependent mechanism. The effect is observed with cell bodies and small neurites rather than their processes. The results exemplify how a structure-encoded synaptogenic protein complex is also used for repulsive cell guidance.

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Crystal structures reveal binding site for Latrophilin on the Teneurin YD shell

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A ternary Latrophilin-Teneurin-FLRT complex forms in vitro and in vivo

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Latrophilin controls cortical migration by binding to Teneurins and FLRTs

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Latrophilin elicits repulsion of cortical cell bodies/small neurites but not axons

Profilin1-Dependent F-Actin Assembly Controls Division of Apical Radial Glia and Neocortex Development

Neocortex development depends on neural stem cell proliferation, cell differentiation, neurogenesis, and neuronal migration. Cytoskeletal regulation is critical for all these processes, but the underlying mechanisms are only poorly understood. We previously implicated the cytoskeletal regulator profilin1 in cerebellar granule neuron migration. Since we found profilin1 expressed throughout mouse neocortex development, we here tested the hypothesis that profilin1 is crucial for neocortex development. We found no evidence for impaired neuron migration or layering in the neocortex of profilin1 mutant mice. However, proliferative activity at basal positions was doubled in the mutant neocortex during mid-neurogenesis, with a drastic and specific increase in basal Pax6+ cells indicative for elevated numbers of basal radial glia (bRG). This was accompanied by transiently increased neurogenesis and associated with mild invaginations resembling rudimentary neocortex folds. Our data are in line with a model in which profilin1-dependent actin assembly controls division of apical radial glia (aRG) and thereby the fate of their progenies. Via this mechanism, profilin1 restricts cell delamination from the ventricular surface and, hence, bRG production and thereby controls neocortex development in mice. Our data support the radial cone hypothesis” claiming that elevated bRG number causes neocortex folds.

Human brain development through the lens of cerebral organoid models

The brain is one of the most complex organs in the body, which emerges from a relatively simple set of basic 'building blocks' during early development according to complex cellular and molecular events orchestrated through a set of inherited instructions. Innovations in stem cell technologies have enabled modelling of neural cells using two- and three-dimensional cultures. In particular, cerebral ('brain') organoids have taken the center stage of brain development models that have the potential for providing meaningful insight into human neurodevelopmental and neurological disorders. We review the current understanding of cellular events during human brain organogenesis, and the events occurring during cerebral organoid differentiation. We highlight the strengths and weaknesses of this experimental model system. In particular, we explain evidence that organoids can mimic many aspects of early neural development, including neural induction, patterning, and broad neurogenesis and gliogenesis programs, offering the opportunity to study genetic regulation of these processes in a human context. Several shortcomings of the current culture methods limit the utility of cerebral organoids to spontaneously give rise to many important cell types, and to model higher order features of tissue organization. We suggest that future studies aim to improve these features in order to make them better models for the study of laminar organization, circuit formation and how disruptions of these processes relate to disease.

A Model of Brain Folding Based on Strong Local and Weak Long-Range Connectivity Requirements

Throughout the animal kingdom, the structure of the central nervous system varies widely from distributed ganglia in worms to compact brains with varying degrees of folding in mammals. The differences in structure may indicate a fundamentally different circuit organization. However, the folded brain most likely is a direct result of mechanical forces when considering that a larger surface area of cortex packs into the restricted volume provided by the skull. Here, we introduce a computational model that instead of modeling mechanical forces relies on dimension reduction methods to place neurons according to specific connectivity requirements. For a simplified connectivity with strong local and weak long-range connections, our model predicts a transition from separate ganglia through smooth brain structures to heavily folded brains as the number of cortical columns increases. The model reproduces experimentally determined relationships between metrics of cortical folding and its pathological phenotypes in lissencephaly, polymicrogyria, microcephaly, autism, and schizophrenia. This suggests that mechanical forces that are known to lead to cortical folding may synergistically contribute to arrangements that reduce wiring. Our model provides a unified conceptual understanding of gyrification linking cellular connectivity and macroscopic structures in large-scale neural network models of the brain.

Construction and reconstruction of brain circuits: normal and pathological axon guidance

Perception of our environment entirely depends on the close interaction between the central and peripheral nervous system. In order to communicate each other, both systems must develop in parallel and in coordination. During development, axonal projections from the CNS as well as the PNS must extend over large distances to reach their appropriate target cells. To do so, they read and follow a series of axon guidance molecules. Interestingly, while these molecules play critical roles in guiding developing axons, they have also been shown to be critical in other major neurodevelopmental processes, such as the migration of cortical progenitors. Currently, a major hurdle for brain repair after injury or neurodegeneration is the absence of axonal regeneration in the mammalian CNS. By contrasts, PNS axons can regenerate. Many hypotheses have been put forward to explain this paradox but recent studies suggest that hacking neurodevelopmental mechanisms may be the key to promote CNS regeneration. Here we provide a seminar report written by trainees attending the second Flagship school held in Alpbach, Austria in September 2018 organized by the International Society for Neurochemistry (ISN) together with the Journal of Neurochemistry (JCN). This advanced school has brought together leaders in the fields of neurodevelopment and regeneration in order to discuss major keystones and future challenges in these respective fields.

Early dorsomedial tissue interactions regulate gyrification of distal neocortex

The extent of neocortical gyrification is an important determinant of a species’ cognitive abilities, yet the mechanisms regulating cortical gyrification are poorly understood. We uncover long-range regulation of this process originating at the telencephalic dorsal midline, where levels of secreted Bmps are maintained by factors in both the neuroepithelium and the overlying mesenchyme. In the mouse, the combined loss of transcription factors Lmx1a and Lmx1b, selectively expressed in the midline neuroepithelium and the mesenchyme respectively, causes dorsal midline Bmp signaling to drop at early neural tube stages. This alters the spatial and temporal Wnt signaling profile of the dorsal midline cortical hem, which in turn causes gyrification of the distal neocortex. Our study uncovers early mesenchymal-neuroepithelial interactions that have long-range effects on neocortical gyrification and shows that lissencephaly in mice is actively maintained via redundant genetic regulation of dorsal midline development and signaling.

The contribution of long-range signaling to cortical gyrification remains poorly understood. In this study, authors demonstrate that the combined genetic loss of transcription factors Lmx1a and Lmx1b, expressed in the telencephalic dorsal midline neuroepithelium and head mesenchyme, respectively, induces gyrification in the mouse neocortex

Genes and Mechanisms Involved in the Generation and Amplification of Basal Radial Glial Cells

The development of the cerebral cortex relies on different types of progenitor cell. Among them, the recently described basal radial glial cell (bRG) is suggested to be of critical importance for the development of the brain in gyrencephalic species. These cells are highly numerous in primate and ferret brains, compared to lissencephalic species such as the mouse in which they are few in number. Their somata are located in basal subventricular zones in gyrencephalic brains and they generally possess a basal process extending to the pial surface. They sometimes also have an apical process directed toward the ventricular surface, similar to apical radial glial cells (aRGs) from which they are derived, and whose somata are found more apically in the ventricular zone. bRGs share similarities with aRGs in terms of gene expression (SOX2, PAX6, and NESTIN), whilst also expressing a range of more specific genes (such as HOPX). In primate brains, bRGs can divide multiple times, self-renewing and/or generating intermediate progenitors and neurons. They display a highly specific cytokinesis behavior termed mitotic somal translocation. We focus here on recently identified molecular mechanisms associated with the generation and amplification of bRGs, including bRG-like cells in the rodent. These include signaling pathways such as the FGF-MAPK cascade, SHH, PTEN/AKT, PDGF pathways, and proteins such as INSM, GPSM2, ASPM, TRNP1, ARHGAP11B, PAX6, and HIF1α. A number of these proteins were identified through transcriptome comparisons in human aRGs vs. bRGs, and validated by modifying their activities or expression levels in the mouse. This latter experiment often revealed enhanced bRG-like cell production, even in some cases generating folds (gyri) on the surface of the mouse cortex. We compare the features of the identified cells and methods used to characterize them in each model. These important data converge to indicate pathways essential for the production and expansion of bRGs, which may help us understand cortical development in health and disease.

Latrophilin's Social Protein Network

Latrophilins (LPHNs) are adhesion GPCRs that are originally discovered as spider's toxin receptors, but are now known to be involved in brain development and linked to several neuronal and non-neuronal disorders. Latrophilins act in conjunction with other cell adhesion molecules and may play a leading role in its network organization. Here, we focus on the main protein partners of latrophilins, namely teneurins, FLRTs and contactins and summarize their respective temporal and spatial expression patterns, links to neurodevelopmental disorders as well as their structural characteristics. We discuss how more recent insights into the separate cell biological functions of these proteins shed light on the central role of latrophilins in this network. We postulate that latrophilins control the refinement of synaptic properties of specific subtypes of neurons, requiring discrete combinations of proteins.

Cross-Talk between Fibroblast Growth Factor Receptors and Other Cell Surface Proteins

Fibroblast growth factors (FGFs) and their receptors (FGFRs) constitute signaling circuits that transmit signals across the plasma membrane, regulating pivotal cellular processes like differentiation, migration, proliferation, and apoptosis. The malfunction of FGFs/FGFRs signaling axis is observed in numerous developmental and metabolic disorders, and in various tumors. The large diversity of FGFs/FGFRs functions is attributed to a great complexity in the regulation of FGFs/FGFRs-dependent signaling cascades. The function of FGFRs is modulated at several levels, including gene expression, alternative splicing, posttranslational modifications, and protein trafficking. One of the emerging ways to adjust FGFRs activity is through formation of complexes with other integral proteins of the cell membrane. These proteins may act as coreceptors, modulating binding of FGFs to FGFRs and defining specificity of elicited cellular response. FGFRs may interact with other cell surface receptors, like G-protein-coupled receptors (GPCRs) or receptor tyrosine kinases (RTKs). The cross-talk between various receptors modulates the strength and specificity of intracellular signaling and cell fate. At the cell surface FGFRs can assemble into large complexes involving various cell adhesion molecules (CAMs). The interplay between FGFRs and CAMs affects cell–cell interaction and motility and is especially important for development of the central nervous system. This review summarizes current stage of knowledge about the regulation of FGFRs by the plasma membrane-embedded partner proteins and highlights the importance of FGFRs-containing membrane complexes in pathological conditions, including cancer.

Using brain organoids to study human neurodevelopment, evolution and disease

The brain is one of the most complex organs, responsible for the advanced intellectual and cognitive ability of humans. Although primates are to some extent capable of performing cognitive tasks, their abilities are less evolved. One of the reasons for this is the vast differences in the brain of humans compared to other mammals, in terms of shape, size and complexity. Such differences make the study of human brain development fascinating. Interestingly, the cerebral cortex is by far the most complex brain region resulting from its selective evolution within mammals over millions of years. Unraveling the molecular and cellular mechanisms regulating brain development, as well as the evolutionary differences seen across species and the need to understand human brain disorders, are some of the reasons why scientists are interested in improving their current knowledge on human corticogenesis. Toward this end, several animal models including primates have been used, however, these models are limited in their extent to recapitulate human-specific features. Recent technological achievements in the field of stem cell research, which have enabled the generation of human models of corticogenesis, called brain or cerebral organoids, are of great importance. This review focuses on the main cellular and molecular features of human corticogenesis and the use of brain organoids to study it. We will discuss the key differences between cortical development in human and nonhuman mammals, the technological applications of brain organoids and the different aspects of cortical development in normal and pathological conditions, which can be modeled using brain organoids. This article is categorized under: Comparative Development and Evolution > Regulation of Organ Diversity Nervous System Development > Vertebrates: General Principles.

Brain organoids: advances, applications and challenges

Brain organoids are self-assembled three-dimensional aggregates generated from pluripotent stem cells with cell types and cytoarchitectures that resemble the embryonic human brain. As such, they have emerged as novel model systems that can be used to investigate human brain development and disorders. Although brain organoids mimic many key features of early human brain development at molecular, cellular, structural and functional levels, some aspects of brain development, such as the formation of distinct cortical neuronal layers, gyrification, and the establishment of complex neuronal circuitry, are not fully recapitulated. Here, we summarize recent advances in the development of brain organoid methodologies and discuss their applications in disease modeling. In addition, we compare current organoid systems to the embryonic human brain, highlighting features that currently can and cannot be recapitulated, and discuss perspectives for advancing current brain organoid technologies to expand their applications.

How Forces Fold the Cerebral Cortex

Improved understanding of the factors that govern folding of the cerebral cortex is desirable for many reasons. The existence of consistent patterns in folding within and between species suggests a fundamental role in brain function. Abnormal folding patterns found in individuals affected by a diverse array of neurodevelopmental disorders underline the clinical relevance of understanding the folding process. Recent experimental and computational efforts to elucidate the biomechanical forces involved in cerebral cortical folding have converged on a consistent approach. Brain growth is modeled with two components: an expanding outer zone, destined to become the cerebral cortex, is mechanically coupled to an inner zone, destined to become white matter, that grows at a slower rate, perhaps in response to stress induced by expansion from the outer layer. This framework is consistent with experimentally observed internal forces in developing brains, and with observations of the folding process in physical models. In addition, computational simulations based on this foundation can produce folding patterns that recapitulate the characteristics of folding patterns found in gyroencephalic brains. This perspective establishes the importance of mechanical forces in our current understanding of how brains fold, and identifies realistic ranges for specific parameters in biophysical models of developing brain tissue. However, further refinement of this approach is needed. An understanding of mechanical forces that arise during brain development and their cellular-level origins is necessary to interpret the consequences of abnormal brain folding and its role in functional deficits as well as neurodevelopmental disease.Dual Perspectives Companion Paper: How Cells Fold the Cerebral Cortex, by Víctor Borrell.

How Cells Fold the Cerebral Cortex

Folding of the cerebral cortex is as highly intriguing as poorly understood. At first sight, this may appear as simple tissue crumpling inside an excessively small cranium, but the process is clearly much more complex and developmentally predetermined. Whereas theoretical modeling supports a critical role for biomechanics, experimental evidence demonstrates the fundamental role of specific progenitor cell types, cellular processes, and genetic programs on cortical folding.Dual Perspectives Companion Paper: How Forces Fold the Cerebral Cortex, by Christopher D. Kroenke and Philip V. Bayly.

Structural brain anomalies in patients with FOXG1 syndrome and in Foxg1+/- mice

Objective

FOXG1 syndrome is a rare neurodevelopmental disorder associated with heterozygous FOXG1 variants or chromosomal microaberrations in 14q12. The study aimed at assessing the scope of structural cerebral anomalies revealed by neuroimaging to delineate the genotype and neuroimaging phenotype associations.

Methods

We compiled 34 patients with a heterozygous (likely) pathogenic FOXG1 variant. Qualitative assessment of cerebral anomalies was performed by standardized re-analysis of all 34 MRI data sets. Statistical analysis of genetic, clinical and neuroimaging data were performed. We quantified clinical and neuroimaging phenotypes using severity scores. Telencephalic phenotypes of adult Foxg1+/- mice were examined using immunohistological stainings followed by quantitative evaluation of structural anomalies.

Results

Characteristic neuroimaging features included corpus callosum anomalies (82%), thickening of the fornix (74%), simplified gyral pattern (56%), enlargement of inner CSF spaces (44%), hypoplasia of basal ganglia (38%), and hypoplasia of frontal lobes (29%). We observed a marked, filiform thinning of the rostrum as recurrent highly typical pattern of corpus callosum anomaly in combination with distinct thickening of the fornix as a characteristic feature. Thickening of the fornices was not reported previously in FOXG1 syndrome. Simplified gyral pattern occurred significantly more frequently in patients with early truncating variants. Higher clinical severity scores were significantly associated with higher neuroimaging severity scores. Modeling of Foxg1 heterozygosity in mouse brain recapitulated the associated abnormal cerebral morphology phenotypes, including the striking enlargement of the fornix.

Interpretation

Combination of specific corpus callosum anomalies with simplified gyral pattern and hyperplasia of the fornices is highly characteristic for FOXG1 syndrome.

A mechanical method of cerebral cortical folding development based on thermal expansion

Cortical folding malformations are associated with several severe neurological disorders, including epilepsy, schizophrenia and autism. However, the mechanism behind cerebral cortical folding development is not yet clear. In this paper, we propose a mechanical method based on thermal expansion to simulate the development of human cerebral cortical folding. The influences of stiffness ratio, growth rate ratio, and initial cortical plate thickness on cortical folding are discussed. The results of our thermal expansion model are consistent with previous studies, indicating that abnormal values of the aforementioned three factors could directly lead to cortical folding malformation in a generally fixed pattern.

Neuronal migration in the CNS during development and disease: insights from in vivo and in vitro models

Neuronal migration is a fundamental process that governs embryonic brain development. As such, mutations that affect essential neuronal migration processes lead to severe brain malformations, which can cause complex and heterogeneous developmental and neuronal migration disorders. Our fragmented knowledge about the aetiology of these disorders raises numerous issues. However, many of these can now be addressed through studies of in vivo and in vitro models that attempt to recapitulate human-specific mechanisms of cortical development. In this Review, we discuss the advantages and limitations of these model systems and suggest that a complementary approach, using combinations of in vivo and in vitro models, will broaden our knowledge of the molecular and cellular mechanisms that underlie defective neuronal positioning in the human cerebral cortex.

Primate neocortex development and evolution: Conserved versus evolved folding

The neocortex, the seat of higher cognitive functions, exhibits a key feature across mammalian species-a highly variable degree of folding. Within the neocortex, two distinct subtypes of cortical areas can be distinguished, the isocortex and the proisocortex. Here, we have compared specific spatiotemporal aspects of folding between the proisocortex and the isocortex in 13 primates, including human, chimpanzee, and various Old World and New World monkeys. We find that folding at the boundaries of the dorsal isocortex and the proisocortex, which gives rise to the cingulate sulcus (CiS) and the lateral fissure (LF), is conserved across the primates studied and is therefore referred to as conserved folding. In contrast, the degree of folding within the dorsal isocortex exhibits huge variation across these primates, indicating that this folding, which gives rise to gyri and sulci, is subject to major changes during primate evolution. We therefore refer to the folding within the dorsal isocortex as evolved folding. Comparison of fetal neocortex development in long-tailed macaque and human reveals that the onset of conserved folding precedes the onset of evolved folding. Moreover, the analysis of infant human neocortex exhibiting lissencephaly, a developmental malformation thought to be mainly due to abnormal neuronal migration, shows that the evolved folding is perturbed more than the conserved folding. Taken together, our study presents a two-step model of folding that pertains to primate neocortex development and evolution. Specifically, our data imply that the conserved folding and the evolved folding constitute two distinct, sequential events.

Differential response of pineal microglia to surgical versus pharmacological stimuli

Microglial cells are one of the interstitial elements of the pineal gland (PG). We recently reported the pattern of microglia colonization and activation, and microglia-Pax6⁺ cell interactions during normal pineal ontogeny. Here, we describe the dynamics of microglia-Pax6⁺ cell associations and interactions after surgical or pharmacological manipulation. In adult rats, the superior cervical ganglia (SCG) were exposed, and either bilaterally excised (SCGx) or decentralized (SCGd). In the SCGx PGs, the density of Iba1⁺ microglia increased after surgery and returned to sham baseline levels 13 days later. Pineal microglia also responded to SCGd, a more subtle denervation. The number of clustered Iba1⁺ /PCNA⁺ /ED1⁺ microglia was higher 4 days after both surgeries compared to the sham-operated group. However, the number of Pax6⁺ /PCNA^- cells and the percentage of Pax6⁺ cells contacted by and/or phagocytosed by microglia increased significantly only after SCGx. Separate groups of rats were treated with either bacterial lipopolysaccharides (LPS) or doxycycline (DOX) to activate or inhibit pineal microglia, respectively. Peripheral LPS administration caused an increase in the number of clustered Iba1⁺ /PCNA⁺ /ED1⁺ microglial cells, and in the percentage of Pax6⁺ cells associated with and/or engulfed by microglia. In the LPS-treated PGs, we also noted an increase in the number of PCNA⁺ cells that were Iba1^- within the microglial cell clusters. The density of Pax6⁺ cells did not change after LPS treatment. DOX administration did not influence the parameters analyzed. These data suggest that pineal microglia are highly receptive cells capable of rapidly responding in a differential manner to surgical and pharmacological stimuli.

Gyrification Needs Correct Sodium Flux!

Channelopathies are disorders that affect the function of ion channels, typically resulting in epilepsy. Smith et al. (2018) discover an unusual association between SCN3A, neuronal migration, and cortical folding, outlining sodium channels as important regulators of brain development.

Cortical progenitor biology: key features mediating proliferation versus differentiation

The cerebral cortex is a highly organized structure whose development depends on diverse progenitor cell types, namely apical radial glia, intermediate progenitors, and basal radial glia cells, which are responsible for the production of the correct neuronal output. In recent years, these progenitor cell types have been deeply studied, particularly basal radial glia and their role in cortical expansion and gyrification. We review here a broad series of factors that regulate progenitor behavior and daughter cell fate. We first describe the different neuronal progenitor types, emphasizing the differences between lissencephalic and gyrencephalic species. We then review key factors shown to influence progenitor proliferation versus differentiation, discussing their roles in progenitor dynamics, neuronal production, and potentially brain size and complexity. Although spindle orientation has been considered a critical factor for mode of division and daughter cell output, we discuss other features that are emerging as crucial for these processes such as organelle and cell cycle dynamics. Additionally, we highlight the importance of adhesion molecules and the polarity complex for correct cortical development. Finally, we briefly discuss studies assessing progenitor multipotency and its possible contribution to the production of specific neuronal populations. This review hence summarizes recent aspects of cortical progenitor cell biology, and pinpoints emerging features critical for their behavior.

Leucine-rich repeat-containing synaptic adhesion molecules as organizers of synaptic specificity and diversity

The brain harbors billions of neurons that form distinct neural circuits with exquisite specificity. Specific patterns of connectivity between distinct neuronal cell types permit the transfer and computation of information. The molecular correlates that give rise to synaptic specificity are incompletely understood. Recent studies indicate that cell-surface molecules are important determinants of cell type identity and suggest that these are essential players in the specification of synaptic connectivity. Leucine-rich repeat (LRR)-containing adhesion molecules in particular have emerged as key organizers of excitatory and inhibitory synapses. Here, we discuss emerging evidence that LRR proteins regulate the assembly of specific connectivity patterns across neural circuits, and contribute to the diverse structural and functional properties of synapses, two key features that are critical for the proper formation and function of neural circuits.

Further analysis of synaptic proteins will provide insights into the functioning of neural circuits and associated brain disorders. The brain houses numerous highly specialized neuron types, which transfer and process information via a complex network of synaptic connections. Every neuron develops its own distinctive synapses with specific functions, but exactly how this is achieved is not clear. Joris de Wit and Anna Schroeder at the VIB Center for Brain and Disease Research in Leuven, Belgium, reviewed recent research into the leucine-rich repeat-containing (LRR) proteins, which are thought to be major organizers of synaptic connectivity and key regulators of healthy neural circuit development. Further investigations into the functionality of LRR proteins in the brain will not only improve understanding of neural circuitry but also provide insights into synaptic impairments in brain disorders like schizophrenia.

Genetic maps and patterns of cerebral cortex folding

Folding of the cerebral cortex during brain development is a complex process that depends on the orchestrated action of a number of factors, including generation and proliferation of basal progenitor cells, and the radial migration of neurons. Patterns of primary cortical folding are stereotyped between individuals and across phylogeny, reflecting a strong genetic regulation of the underlying cellular mechanisms. Here we summarize recent findings on cellular and genetic mechanisms regulating this fascinating process that underlies expansion and functional complexification of the mammalian cerebral cortex.

Novel gene function and regulation in neocortex expansion

The expansion of the neocortex during human evolution is due to changes in our genome that result in increased and prolonged proliferation of neural stem and progenitor cells during neocortex development. Three principal types of such genomic changes can be distinguished, first, novel gene regulation in human, second, novel function in human of genes existing in both human and non-human species, and third, novel, human-specific genes. The latter comprise both, increases in the copy number of genes existing also in non-human species, and the emergence of genes giving rise to unique, human-specific gene products. Examples of all these types of changes in the human genome have been identified, with ARHGAP11B constituting a paradigmatic example of a unique, human-specific protein.

Lkb1 regulates granule cell migration and cortical folding of the cerebellar cortex

Cerebellar growth and foliation require the Hedgehog-driven proliferation of granule cell precursors (GCPs) in the external granule layer (EGL). However, that increased or extended GCP proliferation generally does not elicit ectopic folds suggests that additional determinants control cortical expansion and foliation during cerebellar development. Here, we find that genetic loss of the serine-threonine kinase Liver Kinase B1 (Lkb1) in GCPs increased cerebellar cortical size and foliation independent of changes in proliferation or Hedgehog signaling. This finding is unexpected given that Lkb1 has previously shown to be critical for Hedgehog pathway activation in cultured cells. Consistent with unchanged proliferation rate of GCPs, the cortical expansion of Lkb1 mutants is accompanied by thinning of the EGL. The plane of cell division, which has been implicated in diverse processes from epithelial surface expansions to gyrification of the human cortex, remains unchanged in the mutants when compared to wild-type controls. However, we find that Lkb1 mutants display delayed radial migration of post-mitotic GCPs that coincides with increased cortical size, suggesting that aberrant cell migration may contribute to the cortical expansion and increase foliation. Taken together, our results reveal an important role for Lkb1 in regulating cerebellar cortical size and foliation in a Hedgehog-independent manner.

Unfolding the Folding Problem of the Cerebral Cortex: Movin' and Groovin'

The development of reproducible folding in the gyrencephalic cerebral cortex is a topic of great interest to neuroscientists. In a recent paper in Cell, del Toro et al. (2017) show that changing the adhesive properties of neurons in the normally lissencephalic mouse cortex leads to the formation of stereotyped folding.

Enhancing our brains: Genomic mechanisms underlying cortical evolution

Our most distinguishing higher cognitive functions are controlled by the cerebral cortex. Comparative studies detail abundant anatomical and cellular features unique to the human developing and adult neocortex. Emerging genomic studies have further defined vast differences distinguishing developing human neocortices from related primates. These human-specific changes can affect gene function and/or expression, and result from structural variations such as chromosomal deletions and duplications, or from point mutations in coding and noncoding regulatory regions. Here, we review this rapidly growing field which aims to identify and characterize genetic loci unique to the human cerebral cortex. We catalog known human-specific genomic changes distinct from other primates, including those whose function has been interrogated in animal models. We also discuss how new model systems and technologies such as single cell RNA sequencing, primate iPSCs, and gene editing, are enabling the field to gain unprecedented resolution into function of these human-specific changes. Some neurological disorders are thought to uniquely present in humans, thus reinforcing the need to comprehensively understand human-specific gene expression in the developing brain.

Expression, refolding and spectroscopic characterization of fibronectin type III (FnIII)-homology domains derived from human fibronectin leucine rich transmembrane protein (FLRT)-1, -2, and -3

The fibronectin leucine rich transmembrane (FLRT) protein family consists in humans of 3 proteins, FLRT1, -2, and -3. The FLRT proteins contain two extracellular domains separated by an unstructured linker. The most membrane distal part is a leucine rich repeat (LRR) domain responsible for both cis- and trans-interactions, whereas the membrane proximal part is a fibronectin type III (FnIII) domain responsible for a cis-interaction with members of the fibroblast growth factor receptor 1 (FGFR1) family, which results in FGFR tyrosine kinase activation. Whereas the structures of FLRT LRR domains from various species have been determined, the expression and purification of recombinant FLRT FnIII domains, important steps for further structural and functional characterizations of the proteins, have not yet been described. Here we present a protocol for expressing recombinant FLRT-FnIII domains in inclusion bodies in Escherichia coli. His-tags permitted affinity purification of the domains, which subsequently were refolded on a Ni-NTA agarose column by reducing the concentration of urea. The refolding was confirmed by circular dichroism (CD) and ¹H-NMR. By thermal unfolding experiments we show that a strand-strand cystine bridge has significant effect on the stability of the FLRT FnIII fold. We further show by Surface Plasmon Resonance that all three FnIII domains bind to FGFR1, and roughly estimate a K_d for each domain, all K_ds being in the µM range.