Cadherins in the developing central nervous system: an adhesive code for segmental and functional subdivisions

Adhesion-Based Self-Organization in Tissue Patterning

Since the proposal of the differential adhesion hypothesis, scientists have been fascinated by how cell adhesion mediates cellular self-organization to form spatial patterns during development. The search for molecular tool kits with homophilic binding specificity resulted in a diverse repertoire of adhesion molecules. Recent understanding of the dominant role of cortical tension over adhesion binding redirects the focus of differential adhesion studies to the signaling function of adhesion proteins to regulate actomyosin contractility. The broader framework of differential interfacial tension encompasses both adhesion and nonadhesion molecules, sharing the common function of modulating interfacial tension during cell sorting to generate diverse tissue patterns. Robust adhesion-based patterning requires close coordination between morphogen signaling, cell fate decisions, and changes in adhesion. Current advances in bridging theoretical and experimental approaches present exciting opportunities to understand molecular, cellular, and tissue dynamics during adhesion-based tissue patterning across multiple time and length scales.

Crosstalk between PI3K/AKT/mTOR and WNT/β-Catenin signaling in GBM - Could combination therapy checkmate the collusion?

Glioblastoma multiforme is one of the calamitous primary glial brain tumors with extensive heterogeneity at cellular and molecular levels. While maximal surgical resection trailed by radio and chemotherapy employing temozolomide remains the gold-standard treatment for malignant glioma patients, the overall prognosis remains dismal and there exists an unmet need for effective therapeutic strategies. In this context, we hypothesize that proper understanding of signaling pathways responsible for glioblastoma multiforme proliferation would be the first trump card while searching for novel targeted therapies. Among the pathways aberrantly activated, PI3K/AKT/mTOR is the most significant pathway, that is clinically implicated in malignancies such as high-grade glioma. Further, the WNT/β-Catenin cascade is well-implicated in several malignancies, while its role in regulating glioma pathogenesis has only emerged recently. Nevertheless, oncogenic activation of both these pathways is a frequent event in malignant glioma that facilitates tumor proliferation, stemness and chemo-resistance. Recently, it has been reported that the cross-talk of PI3K/AKT/mTOR pathway with multiple signaling pathways could promote glioma progression and reduce the sensitivity of glioma cells to the standard therapy. However, very few studies had focused on the relationship between PI3K/AKT/mTOR and WNT/β-Catenin pathways in glioblastoma multiforme. Interestingly, in homeostatic and pathologic circumstances, both these pathways depict fine modulation and are connected at multiple levels by upstream and downstream effectors. Thus, gaining deep insights on the collusion between these pathways would help in discovering unique therapeutic targets for glioblastoma multiforme management. Hence, the current review aims to address, "the importance of inter-play between PI3K/AKT/mTOR and WNT/β-Catenin pathways", and put forward, "the possibility of combinatorially targeting them", for glioblastoma multiforme treatment enhancement.

Cadherin Expression and EMT: A Focus on Gliomas

Cadherins are calcium-binding proteins with a pivotal role in cell adhesion and tissue homeostasis. The cadherin-dependent mechanisms of cell adhesion and migration are exploited by cancer cells, contributing to tumor invasiveness and dissemination. In particular, cadherin switch is a hallmark of epithelial to mesenchymal transition, a complex development process vastly described in the progression of most epithelial cancers. This is characterized by drastic changes in cell polarity, adhesion, and motility, which lead from an E-cadherin positive differentiated epithelial state into a dedifferentiated mesenchymal-like state, prone to metastization and defined by N-cadherin expression. Although vastly explored in epithelial cancers, how these mechanisms contribute to the pathogenesis of other non-epithelial tumor types is poorly understood. Herein, the current knowledge on cadherin expression in normal development in parallel to tumor pathogenesis is reviewed, focusing on epithelial to mesenchymal transition. Emphasis is taken in the unascertained cadherin expression in CNS tumors, particularly in gliomas, where the potential contribution of an epithelial-to-mesenchymal-like process to glioma genesis and how this may be associated with changes in cadherin expression is discussed.

Differential Spatiotemporal Expression of Type I and Type II Cadherins Associated With the Segmentation of the Central Nervous System and Formation of Brain Nuclei in the Developing Mouse

Type I and type II classical cadherins comprise a family of cell adhesion molecules that regulate cell sorting and tissue separation by forming specific homo and heterophilic bonds. Factors that affect cadherin-mediated cell-cell adhesion include cadherin binding affinity and expression level. This study examines the expression pattern of type I cadherins (Cdh1, Cdh2, Cdh3, and Cdh4), type II cadherins (Cdh6, Cdh7, Cdh8, Cdh9, Cdh10, Cdh11, Cdh12, Cdh18, Cdh20, and Cdh24), and the atypical cadherin 13 (Cdh13) during distinct morphogenetic events in the developing mouse central nervous system from embryonic day 11.5 to postnatal day 56. Cadherin mRNA expression levels obtained from in situ hybridization experiments carried out at the Allen Institute for Brain Science (https://alleninstitute.org/) were retrieved from the Allen Developing Mouse Brain Atlas. Cdh2 is the most abundantly expressed type I cadherin throughout development, while Cdh1, Cdh3, and Cdh4 are expressed at low levels. Type II cadherins show a dynamic pattern of expression that varies between neuroanatomical structures and developmental ages. Atypical Cdh13 expression pattern correlates with Cdh2 in abundancy and localization. Analyses of cadherin-mediated relative adhesion estimated from their expression level and binding affinity show substantial differences in adhesive properties between regions of the neural tube associated with the segmentation along the anterior–posterior axis. Differences in relative adhesion were also observed between brain nuclei in the developing subpallium (basal ganglia), suggesting that differential cell adhesion contributes to the segregation of neuronal pools. In the adult cerebral cortex, type II cadherins Cdh6, Cdh8, Cdh10, and Cdh12 are abundant in intermediate layers, while Cdh11 shows a gradated expression from the deeper layer 6 to the superficial layer 1, and Cdh9, Cdh18, and Cdh24 are more abundant in the deeper layers. Person’s correlation analyses of cadherins mRNA expression patterns between areas and layers of the cerebral cortex and the nuclei of the subpallium show significant correlations between certain cortical areas and the basal ganglia. The study shows that differential cadherin expression and cadherin-mediated adhesion are associated with a wide range of morphogenetic events in the developing central nervous system including the organization of neurons into layers, the segregation of neurons into nuclei, and the formation of neuronal circuits.

Redundant type II cadherins define neuroepithelial cell states for cytoarchitectonic robustness

Individual cell shape and integrity must precisely be orchestrated during morphogenesis. Here, we determine function of type II cadherins, Cdh6, Cdh8, and Cdh11, whose expression combinatorially demarcates the mouse neural plate/tube. While CRISPR/Cas9-based single type II cadherin mutants show no obvious phenotype, Cdh6/8 double knockout (DKO) mice develop intermingled forebrain/midbrain compartments as these two cadherins' expression opposes at the nascent boundary. Cdh6/8/11 triple, Cdh6/8 or Cdh8/11 DKO mice further cause exencephaly just within the cranial region where mutated cadherins' expression merges. In the Cdh8/11 DKO midbrain, we observe less-constricted apical actin meshwork, ventrally-directed spreading, and occasional hyperproliferation among dorsal neuroepithelial cells as origins for exencephaly. These results provide rigid evidence that, by conferring distinct adhesive codes to each cell, redundant type II cadherins serve essential and shared roles in compartmentalization and neurulation, both of which proceed under the robust control of the number, positioning, constriction, and fluidity of neuroepithelial cells.

Neural Cadherin Plays Distinct Roles for Neuronal Survival and Axon Growth under Different Regenerative Conditions

Growing axons in the CNS often migrate along specific pathways to reach their targets. During embryonic development, this migration is guided by different types of cell adhesion molecules (CAMs) present on the surface of glial cells or other neurons, including the neural cadherin (NCAD). Axons in the adult CNS can be stimulated to regenerate, and travel long distances. Crucially, however, while a few axons are guided effectively through the injured nerve under certain conditions, most axons never migrate properly. The molecular underpinnings of the variable growth, and the glial CAMs that are responsible for CNS axon regeneration remain unclear. Here we used optic nerve crush to demonstrate that NCAD plays multifaceted functions in facilitating CNS axon regeneration. Astrocyte-specific deletion of NCAD dramatically decreases regeneration induced by phosphatase and tensin homolog (PTEN) ablation in retinal ganglion cells (RGCs). Consistent with NCAD’s tendency to act as homodimers, deletion of NCAD in RGCs also reduces regeneration. Deletion of NCAD in astrocytes neither alters RGCs’ mammalian target of rapamycin complex 1 (mTORC1) activity nor lesion size, two factors known to affect regeneration. Unexpectedly, however, we find that NCAD deletion in RGCs reduces PTEN-deletion-induced RGC survival. We further show that NCAD deletion, in either astrocytes or RGCs, has negligible effects on the regeneration induced by ciliary neurotrophic factor (CNTF), suggesting that other CAMs are critical under this regenerative condition. Consistent with this notion, CNTF induces expression various integrins known to mediate cell adhesion. Together, our study reveals multilayered functions of NCAD and a molecular basis of variability in guided axon growth.

Neural defects caused by total and Wnt1-Cre mediated ablation of p120ctn in mice

Background

p120 catenin (p120ctn) is an important component in the cadherin-catenin cell adhesion complex because it stabilizes cadherin-mediated intercellular junctions. Outside these junctions, p120ctn is actively involved in the regulation of small GTPases of the Rho family, in actomyosin dynamics and in transcription regulation. We and others reported that loss of p120ctn in mouse embryos results in an embryonic lethal phenotype, but the exact developmental role of p120ctn during brain formation has not been reported.

Results

We combined floxed p120ctn mice with Del-Cre or Wnt1-Cre mice to deplete p120ctn from either all cells or specific brain and neural crest cells. Complete loss of p120ctn in mid-gestation embryos resulted in an aberrant morphology, including growth retardation, failure to switch from lordotic to fetal posture, and defective neural tube formation and neurogenesis. By expressing a wild-type p120ctn from the ROSA26 locus in p120ctn-null mouse embryonic stem cells, we could partially rescue neurogenesis. To further investigate the developmental role of p120ctn in neural tube formation, we generated conditional p120ctn^fl/fl;Wnt1Cre knockout mice. p120ctn deletion in Wnt1-expressing cells resulted in neural tube closure defects (NTDs) and craniofacial abnormalities. These defects could not be correlated with misregulation of brain marker genes or cell proliferation. In contrast, we found that p120ctn is required for proper expression of the cell adhesion components N-cadherin, E-cadherin and β-catenin, and of actin-binding proteins cortactin and Shroom3 at the apical side of neural folds. This region is of critical importance for closure of neural folds. Surprisingly, the lateral side of mutant neural folds showed loss of p120ctn, but not of N-cadherin, β-catenin or cortactin.

Conclusions

These results indicate that p120ctn is required for neurogenesis and neurulation. Elimination of p120ctn in cells expressing Wnt1 affects neural tube closure by hampering correct formation of specific adhesion and actomyosin complexes at the apical side of neural folds. Collectively, our results demonstrate the crucial role of p120ctn during brain morphogenesis.

Molecular mechanisms regulating synaptic specificity and retinal circuit formation

The central nervous system (CNS) is composed of precisely assembled circuits which support a variety of physiological functions and behaviors. These circuits include multiple subtypes of neurons with unique morphologies, electrical properties, and molecular identities. How these component parts are precisely wired-up has been a topic of great interest to the field of developmental neurobiology and has implications for our understanding of the etiology of many neurological disorders and mental illnesses. To date, many molecules involved in synaptic choice and specificity have been identified, including members of several families of cell-adhesion molecules (CAMs), which are cell-surface molecules that mediate cell-cell contacts and subsequent intracellular signaling. One favored hypothesis is that unique expression patterns of CAMs define specific neuronal subtype populations and determine compatible pre- and postsynaptic neuronal partners based on the expression of these unique CAMs. The mouse retina has served as a beautiful model for investigations into mammalian CAM interactions due to its well-defined neuronal subtypes and distinct circuits. Moreover, the retina is readily amenable to visualization of circuit organization and electrophysiological measurement of circuit function. The advent of recent genetic, genomic, and imaging technologies has opened the field up to large-scale, unbiased approaches for identification of new molecular determinants of synaptic specificity. Thus, building on the foundation of work reviewed here, we can expect rapid expansion of the field, harnessing the mouse retina as a model to understand the molecular basis for synaptic specificity and functional circuit assembly. This article is categorized under: Nervous System Development > Vertebrates: General Principles Nervous System Development > Vertebrates: Regional Development.

Enhanced cell-cell contact stability and decreased N-cadherin-mediated migration upon fibroblast growth factor receptor-N-cadherin cross talk

N-cadherin adhesion has been reported to enhance cancer and neuronal cell migration either by mediating actomyosin-based force transduction or initiating fibroblast growth factor receptor (FGFR)-dependent biochemical signalling. Here we show that FGFR1 reduces N-cadherin-mediated cell migration. Both proteins are co-stabilised at cell-cell contacts through direct interaction. As a consequence, cell adhesion is strengthened, limiting the migration of cells on N-cadherin. Both the inhibition of migration and the stabilisation of cell adhesions require the FGFR activity stimulated by N-cadherin engagement. FGFR1 stabilises N-cadherin at the cell membrane through a pathway involving Src and p120. Moreover, FGFR1 stimulates the anchoring of N-cadherin to actin. We found that the migratory behaviour of cells depends on an optimum balance between FGFR-regulated N-cadherin adhesion and actin dynamics. Based on these findings we propose a positive feed-back loop between N-cadherin and FGFR at adhesion sites limiting N-cadherin-based single-cell migration.

N-Cadherin Orchestrates Self-Organization of Neurons within a Columnar Unit in the Drosophila Medulla

Columnar structure is a basic unit of the brain, but the mechanism underlying its development remains largely unknown. The medulla, the largest ganglion of the Drosophila melanogaster visual center, provides a unique opportunity to reveal the mechanisms of 3D organization of the columns. In this study, using N-cadherin (Ncad) as a marker, we reveal the donut-like columnar structures along the 2D layer in the larval medulla that evolves to form three distinct layers in pupal development. Column formation is initiated by three core neurons, R8, R7, and Mi1, which establish distinct concentric domains within a column. We demonstrate that Ncad-dependent relative adhesiveness of the core columnar neurons regulates their relative location within a column along a 2D layer in the larval medulla according to the differential adhesion hypothesis. We also propose the presence of mutual interactions among the three layers during formation of the 3D structures of the medulla columns.SIGNIFICANCE STATEMENT The columnar structure is a basic unit of the brain, but its developmental mechanism remains unknown. The medulla, the largest ganglion of the fly visual center, provides a unique opportunity to reveal the mechanisms of 3D organization of the columns. We reveal that column formation is initiated by three core neurons that establish distinct concentric domains within a column. We demonstrate the in vivo evidence of N-cadherin-dependent differential adhesion among the core columnar neurons within a column along a 2D layer in the larval medulla. The 2D larval columns evolve to form three distinct layers in the pupal medulla. We propose the presence of mutual interactions among the three layers during formation of the 3D structures of the medulla columns.

Cellular organization and boundary formation in craniofacial development

Craniofacial morphogenesis is a highly dynamic process that requires changes in the behaviors and physical properties of cells in order to achieve the proper organization of different craniofacial structures. Boundary formation is a critical process in cellular organization, patterning, and ultimately tissue separation. There are several recurring cellular mechanisms through which boundary formation and cellular organization occur including, transcriptional patterning, cell segregation, cell adhesion and migratory guidance. Disruption of normal boundary formation has dramatic morphological consequences, and can result in human craniofacial congenital anomalies. In this review we discuss boundary formation during craniofacial development, specifically focusing on the cellular behaviors and mechanisms underlying the self-organizing properties that are critical for craniofacial morphogenesis.

Probing Cadherin Interactions in Zebrafish with E- and N-Cadherin Missense Mutants

Cadherins are cell adhesion molecules that regulate numerous adhesive interactions during embryonic development and adult life. Consistent with these functions, when their expression goes astray cells lose their normal adhesive properties resulting in defective morphogenesis, disease, and even metastatic cancer. In general, classical cadherins exert their effect by homophilic interactions via their five characteristic extracellular (EC) repeats. The EC1 repeat provides the mechanism for cadherins to dimerize with each other whereas the EC2 repeat may facilitate dimerization. Less is known about the other EC repeats. Here, we show that a zebrafish missense mutation in the EC5 repeat of N-cadherin is a dominant gain-of-function mutation and demonstrate that this mutation alters cell adhesion almost to the same degree as a zebrafish missense mutation in the EC1 repeat of N-cadherin. We also show that zebrafish E- and N-cadherin dominant gain-of-function missense mutations genetically interact. Perturbation of cell adhesion in embryos that are heterozygous mutant at both loci is similar to that observed in single homozygous mutants. Introducing an E-cadherin EC5 missense allele into the homozygous N-cadherin EC1 missense mutant more radically affects morphogenesis, causing synergistic phenotypes consistent with interdependent functions being disrupted. Our studies indicate that a functional EC5 repeat is critical for cadherin-mediated cell affinity, suggesting that its role may be more important than previously thought. These results also suggest the possibility that E- and N-cadherin have heterophilic interactions during early morphogenesis of the embryo; interactions that might help balance the variety of cell affinities needed during embryonic development.

CD226 deficiency improves cognitive functions and ameliorates anxiety-like behaviors in mice

BACKGROUND

CD226 is a cell surface adhesion molecule expressed in the immune system and central nervous system. Although the role of CD226 in the function of immune cells has been well studied, there has been no report on the potential functional significance of CD226 in neural cells.

METHODS

We investigated the role of CD226 on the cognitive function and behaviors using CD226 knockout (CD226KO) and wild-type mice. The spatial learning and memory were characterized using Morris water maze test, and the behaviors were evaluated using open field and elevated plus maze tests. IL-10 expression in the hippocampus was measured using RT-PCR and ELISA.

RESULTS

The results showed that CD226KO mice displayed increased spatial learning and memory than the wild-type controls. We also found that genetic deletion of CD226 resulted in decreased anxiety-like behaviors. In addition, the hippocampal expression level of IL-10 was increased in the CD226KO mice compared with the WT mice.

CONCLUSIONS

Our findings suggest that CD226 plays an important role in the modulation of cognition and anxiety in mice.

Heterophilic Type II Cadherins Are Required for High-Magnitude Synaptic Potentiation in the Hippocampus

Hippocampal CA3 neurons form synapses with CA1 neurons in two layers, stratum oriens (SO) and stratum radiatum (SR). Each layer develops unique synaptic properties but molecular mechanisms that mediate these differences are unknown. Here, we show that SO synapses normally have significantly more mushroom spines and higher-magnitude long-term potentiation (LTP) than SR synapses. Further, we discovered that these differences require the Type II classic cadherins, cadherins-6, -9, and -10. Though cadherins typically function via trans-cellular homophilic interactions, our results suggest presynaptic cadherin-9 binds postsynaptic cadherins-6 and -10 to regulate mushroom spine density and high-magnitude LTP in the SO layer. Loss of these cadherins has no effect on the lower-magnitude LTP typically observed in the SR layer, demonstrating that cadherins-6, -9, and -10 are gatekeepers for high-magnitude LTP. Thus, Type II cadherins may uniquely contribute to the specificity and strength of synaptic changes associated with learning and memory.

Pax3 overexpression induces cell aggregation and perturbs commissural axon projection during embryonic spinal cord development

Pax3 is a transcription factor that belongs to the paired box family. In the developing spinal cord it is expressed in the dorsal commissural neurons, which project ascending axons contralaterally to form proper spinal cord-brain circuitry. While it has been shown that Pax3 induces cell aggregation in vitro, little is known about the role of Pax3 in cell aggregation and spinal circuit formation in vivo. We have reported that Pax3 is involved in neuron differentiation and that its overexpression induces ectopic cadherin-7 expression. In this study we report that Pax3 overexpression also induces cell aggregation in vivo. Tissue sections and open book preparations revealed that Pax3 overexpression prevents commissural axons from projecting to the contralateral side of the spinal cord. Cells overexpressing Pax3 aggregated in cell clusters that contained shortened neurites with perturbed axon growth and elongation. Pax3-specific shRNA partially rescued the morphological change induced by Pax3 overexpression in vivo. Our results indicate that the normal expression of Pax3 is necessary for proper axonal pathway finding and commissural axon projection. In conclusion, Pax3 regulates neural circuit formation during embryonic development. J. Comp. Neurol. 525:1618-1632, 2017. © 2016 Wiley Periodicals, Inc.

Characterizing PCDH19 in human induced pluripotent stem cells (iPSCs) and iPSC-derived developing neurons: emerging role of a protein involved in controlling polarity during neurogenesis

PCDH19 (Protocadherin 19), a member of the cadherin superfamily, is involved in the pathogenic mechanism of an X-linked model of neurological disease. The biological function of PCHD19 in human neurons and during neurogenesis is currently unknown. Therefore, we decided to use the model of the induced pluripotent stem cells (iPSCs) to characterize the location and timing of expression of PCDH19 during cortical neuronal differentiation. Our data show that PCDH19 is expressed in pluripotent cells before differentiation in a homogeneous pattern, despite its localization is often limited to one pole of the cell. During neuronal differentiation, positional information on the progenitor cells assumes an important role in acquiring polarization. The proper control of the cell orientation ensures a fine balancing between symmetric (giving rise to two progenitor sister cells) versus asymmetric (giving rise to one progenitor cell and one newborn neuron) division. This process results in the polar organization of the neural tube with a lumen indicating the basal part of the polarized neuronal progenitor cell; in the iPSC model the cells are organized in the ‘neural rosette’ and interestingly, PCDH19 is located at the center of the rosette, with other well-known markers of the lumen (N-cadherin and ZO-1). These data suggest that PCDH19 has a role in instructing the apico-basal polarity of the progenitor cells, thus regulating the development of a properly organized human brain.

Epb41l5 competes with Delta as a substrate for Mib1 to coordinate specification and differentiation of neurons

We identified Erythrocyte membrane protein band 4.1-like 5 (Epb41l5) as a substrate for the E3 ubiquitin ligase Mind bomb 1 (Mib1), which is essential for activation of Notch signaling. Although loss of Epb41l5 does not significantly alter the pattern of neural progenitor cells (NPCs) specified as neurons at the neural plate stage, it delays their delamination and differentiation after neurulation when NPCs normally acquire organized apical junctional complexes (AJCs) in the zebrafish hindbrain. Delays in differentiation are reduced by knocking down N-cadherin, a manipulation expected to help destabilize adherens junctions (AJs). This suggested that delays in neuronal differentiation in epb41l5-deficient embryos are related to a previously described role for Epb41l5 in facilitating disassembly of cadherin-dependent AJCs. Mib1 ubiquitylates Epb41l5 to promote its degradation. DeltaD can compete with Epb41l5 to reduce Mib1-dependent Epb41l5 degradation. In this context, increasing the number of NPCs specified to become neurons, i.e. cells expressing high levels of DeltaD, stabilizes Epb41l5 in the embryo. Together, these observations suggest that relatively high levels of Delta stabilize Epb41l5 in NPCs specified as neurons. This, we suggest, helps coordinate NPC specification with Epb41l5-dependent delamination and differentiation as neurons.

N-Cadherin and Fibroblast Growth Factor Receptors crosstalk in the control of developmental and cancer cell migrations

Cell migrations are diverse. They constitutemajor morphogenetic driving forces during embryogenesis, but they contribute also to the loss of tissue homeostasis and cancer growth. Capabilities of cells to migrate as single cells or as collectives are controlled by internal and external signalling, leading to the reorganisation of their cytoskeleton as well as by the rebalancing of cell-matrix and cell-cell adhesions. Among the genes altered in numerous cancers, cadherins and growth factor receptors are of particular interest for cell migration regulation. In particular, cadherins such as N-cadherin and a class of growth factor receptors, namely FGFRs cooperate to regulate embryonic and cancer cell behaviours. In this review, we discuss on reciprocal crosstalk between N-cadherin and FGFRs during cell migration. Finally, we aim at clarifying the synergy between N-cadherin and FGFR signalling that ensure cellular reorganization during cell movements, mainly during cancer cell migration and metastasis but also during developmental processes.

A new theory of development: the generation of complexity in ontogenesis

Today there is a very wide consensus on the idea that embryonic development is the result of a genetic programme and of epigenetic processes. Many models have been proposed in this theoretical framework to account for the various aspects of development, and virtually all of them have one thing in common: they do not acknowledge the presence of organic codes (codes between organic molecules) in ontogenesis. Here it is argued instead that embryonic development is a convergent increase in complexity that necessarily requires organic codes and organic memories, and a few examples of such codes are described. This is the code theory of development, a theory that was originally inspired by an algorithm that is capable of reconstructing structures from incomplete information, an algorithm that here is briefly summarized because it makes it intuitively appealing how a convergent increase in complexity can be achieved. The main thesis of the new theory is that the presence of organic codes in ontogenesis is not only a theoretical necessity but, first and foremost, an idea that can be tested and that has already been found to be in agreement with the evidence.

Specification of synaptic connectivity by cell surface interactions

The molecular diversification of cell surface molecules has long been postulated to impart specific surface identities on neuronal cell types. The existence of unique cell surface identities would allow neurons to distinguish one another and connect with their appropriate target cells. Although progress has been made in identifying cell type-specific surface molecule repertoires and in characterizing their extracellular interactions, determining how this molecular diversity contributes to the precise wiring of neural circuitry has proven challenging. Here, we review the role of the cadherin, neurexin, immunoglobulin and leucine-rich repeat protein superfamilies in the specification of connectivity. The emerging evidence suggests that the concerted actions of these proteins may critically contribute to the assembly of neural circuits.

Protocadherins and hypothalamic development: do they play an unappreciated role?

Normal brain development requires coordinated cell movements at precise times. It has long been established that cell-cell adhesion proteins of the cadherin superfamily are involved in the adhesion and sorting of cells during tissue morphogenesis. In the present review, we focus on protocadherins, which form the largest subfamily of the cadherin superfamily and mediate homophilic cell-cell adhesion in the developing brain. These molecules are highly expressed during neural development and the exact roles that they play are still emerging. Although, historically, protocadherins were considered to provide mechanical and chemical connections between adjacent cells, recent research suggests that they may also serve as molecular identity markers of neurones to help guide cell recognition and sorting, cell migration, outgrowth of neuronal processes, and synapse formation. This phenomenon of single cell diversity stems, in part, from the vast variation in protein structure, genomic organisation and molecular function of the protocadherins. Although expression profiles and genetic manipulations have provided evidence for the role of protocadherins in the developing brain, we have only begun to construct a complete understanding of protocadherin function. We examine our current understanding of how protocadherins influence brain development and discuss the possible roles for this large superfamily within the hypothalamus. We conclude that further research into these underappreciated but vitally important genes will shed insight into hypothalamic development and perhaps the underlying aetiology of neuroendocrine disorders.

Differential expression of protocadherin-19, protocadherin-17, and cadherin-6 in adult zebrafish brain

Cell adhesion molecule cadherins play important roles in both development and maintenance of adult structures. Most studies on cadherin expression have been carried out in developing organisms, but information on cadherin distribution in adult vertebrate brains is limited. In this study we used in situ hybridization to examine mRNA expression of three cadherins, protocadherin-19, protocadherin-17, and cadherin-6 in adult zebrafish brain. Each cadherin exhibits a distinct expression pattern in the fish brain, with protocadherin-19 and protocadherin-17 showing much wider and stronger expression than that of cadherin-6. Both protocadherin-19 and protocadherin-17-expressing cells occur throughout the brain, with strong expression in the ventromedial telencephalon, periventricular regions of the thalamus and anterior hypothalamus, stratum periventriculare of the optic tectum, dorsal tegmental nucleus, granular regions of the cerebellar body and valvula, and superficial layers of the facial and vagal lobes. Numerous sensory structures (e.g., auditory, gustatory, lateral line, olfactory, and visual nuclei) and motor nuclei (e.g., oculomotor, trochlear, trigeminal motor, abducens, and vagal motor nuclei) contain protocadherin-19 and/or protocadherin-17-expressing cell. Expression of these two protocadherins is similar in the ventromedial telencephalon, thalamus, hypothalamus, facial, and vagal lobes, but substantially different in the dorsolateral telencephalon, intermediate layers of the optic tectum, and cerebellar valvula. In contrast to the two protocadherins, cadherin-6 expression is much weaker and limited in the adult fish brain.

Cadherins mediate sequential roles through a hierarchy of mechanisms in the developing mammillary body

Expression of intricate combinations of cadherins (a family of adhesive membrane proteins) is common in the developing central nervous system. On this basis, a combinatorial cadherin code has long been proposed to underlie neuronal sorting and to be ultimately responsible for the layers, columns and nuclei of the brain. However, experimental proof of this particular function of cadherins has proven difficult to obtain and the question is still not clear. Alternatively, non-specific, non-combinatorial, purely quantitative adhesive differentials have been proposed to explain neuronal sorting in the brain. Do cadherin combinations underlie brain cytoarchitecture? We approached this question using as model a well-defined forebrain nucleus, the mammillary body (MBO), which shows strong, homogeneous expression of one single cadherin (Cdh11) and patterned, combinatorial expression of Cdh6, −8 and −10. We found that, besides the known combinatorial Cdh pattern, MBO cells are organized into a second, non-overlapping pattern grouping neurons with the same date of neurogenesis. We report that, in the Foxb1 mouse mutant, Cdh11 expression fails to be maintained during MBO development. This disrupted the combination-based as well as the birthdate-based sorting in the mutant MBO. In utero RNA interference (RNAi) experiments knocking down Cdh11 in MBO-fated migrating neurons at one specific age showed that Cdh11 expression is required for chronological entrance in the MBO. Our results suggest that neuronal sorting in the developing MBO is caused by adhesion-based, non-combinatorial mechanisms that keep neurons sorted according to birthdate information (possibly matching them to target neurons chronologically sorted in the same manner). Non-specific adhesion mechanisms would also prevent cadherin combinations from altering the birthdate-based sorting. Cadherin combinations would presumably act later to support specific synaptogenesis through specific axonal fasciculation and final target recognition.

The construction of common and specific significance subnetworks of Alzheimer's disease from multiple brain regions

Alzheimer's disease (AD) is a progressively and fatally neurodegenerative disorder and leads to irreversibly cognitive and memorial damage in different brain regions. The identification and analysis of the dysregulated pathways and subnetworks among affected brain regions will provide deep insights for the pathogenetic mechanism of AD. In this paper, commonly and specifically significant subnetworks were identified from six AD brain regions. Protein-protein interaction (PPI) data were integrated to add molecular biological information to construct the functional modules of six AD brain regions by Heinz algorithm. Then, the simulated annealing algorithm based on edge weight is applied to predicting and optimizing the maximal scoring networks for common and specific genes, respectively, which can remove the weak interactions and add the prediction of strong interactions to increase the accuracy of the networks. The identified common subnetworks showed that inflammation of the brain nerves is one of the critical factors of AD and calcium imbalance may be a link among several causative factors in AD pathogenesis. In addition, the extracted specific subnetworks for each brain region revealed many biologically functional mechanisms to understand AD pathogenesis.

Subventricular zone progenitors in time and space: generating neuronal diversity

The adult mammalian brain harbors a population of cells around their lateral ventricles capable of giving rise to new neurons throughout life. The so-called subventricular zone (SVZ) is a heterogeneous germinative niche in regard to the neuronal types it generates. SVZ progenitors give rise to different olfactory bulb (OB) interneuron types in accordance to their position along the ventricles. Here, I review data showing the difference between progenitors located along different parts of the SVZ axes and ages. I also discuss possible mechanisms for the origin of this diversity.

Type II cadherins guide assembly of a direction-selective retinal circuit

Complex retinal circuits process visual information and deliver it to the brain. Few molecular determinants of synaptic specificity in this system are known. Using genetic and optogenetic methods, we identified two types of bipolar interneurons that convey visual input from photoreceptors to a circuit that computes the direction in which objects are moving. We then sought recognition molecules that promote selective connections of these cells with previously characterized components of the circuit. We found that the type II cadherins, cdh8 and cdh9, are each expressed selectively by one of the two bipolar cell types. Using loss- and gain-of-function methods, we showed that they are critical determinants of connectivity in this circuit and that perturbation of their expression leads to distinct defects in visually evoked responses. Our results reveal cellular components of a retinal circuit and demonstrate roles of type II cadherins in synaptic choice and circuit function.

Classic Cadherins Mediate Selective Intracortical Circuit Formation in the Mouse Neocortex

Understanding the molecular mechanisms underlying the formation of selective intracortical circuitry is one of the important questions in neuroscience research. "Barrel nets" are recently identified intracortical axonal trajectories derived from layer 2/3 neurons in layer 4 of the primary somatosensory (barrel) cortex. Axons of layer 2/3 neurons are preferentially distributed in the septal regions of layer 4 of the barrel cortex, where they show whisker-related patterns. Because cadherins have been viewed as potential candidates that mediate the formation of selective neuronal circuits, here we examined the role of cadherins in the formation of barrel nets. We disrupted the function of cadherins by expressing dominant-negative cadherin (dn-cadherin) using in utero electroporation and found that barrel nets were severely disrupted. Confocal microscopic analysis revealed that expression of dn-cadherin reduced the density of axons in septal regions in layer 4 of the barrel cortex. We also found that cadherins were important for the formation, rather than the maintenance, of barrel nets. Our results uncover an important role of cadherins in the formation of local intracortical circuitry in the neocortex.

Mapping arealisation of the visual cortex of non-primate species: lessons for development and evolution

The integration of the visual stimulus takes place at the level of the neocortex, organized in anatomically distinct and functionally unique areas. Primates, including humans, are heavily dependent on vision, with approximately 50% of their neocortical surface dedicated to visual processing and possess many more visual areas than any other mammal, making them the model of choice to study visual cortical arealisation. However, in order to identify the mechanisms responsible for patterning the developing neocortex, specifying area identity as well as elucidate events that have enabled the evolution of the complex primate visual cortex, it is essential to gain access to the cortical maps of alternative species. To this end, species including the mouse have driven the identification of cellular markers, which possess an area-specific expression profile, the development of new tools to label connections and technological advance in imaging techniques enabling monitoring of cortical activity in a behaving animal. In this review we present non-primate species that have contributed to elucidating the evolution and development of the visual cortex. We describe the current understanding of the mechanisms supporting the establishment of areal borders during development, mainly gained in the mouse thanks to the availability of genetically modified lines but also the limitations of the mouse model and the need for alternate species.

Possible roles of LI-Cadherin in the formation and maintenance of the intestinal epithelial barrier

LI-cadherin belongs to the so called 7D-cadherins, exceptional members of the cadherin superfamily which are characterized by seven extracellular cadherin repeats and a small cytosolic domain. Under physiological conditions LI-cadherin is expressed in the intestine and colon in human and mouse and in the rat also in hepatocytes. LI-cadherin was shown to act as a functional Ca²⁺-dependent adhesion molecule, linking neighboring cells and a lot of biophysical and biochemical parameters were determined in the last time. It is also known that dysregulated LI-cadherin expression can be found in a variety of diseases. Although there are several hypothesis and theoretical models concerning the function of LI-cadherin, the physiological role of LI-cadherin is still enigmatic.

N-cadherin sustains motility and polarity of future cortical interneurons during tangential migration

In the developing brain, cortical GABAergic interneurons migrate long distances from the medial ganglionic eminence (MGE) in which they are generated, to the cortex in which they settle. MGE cells express the cell adhesion molecule N-cadherin, a homophilic cell-cell adhesion molecule that regulates numerous steps of brain development, from neuroepithelium morphogenesis to synapse formation. N-cadherin is also expressed in embryonic territories crossed by MGE cells during their migration. In this study, we demonstrate that N-cadherin is a key player in the long-distance migration of future cortical interneurons. Using N-cadherin-coated substrate, we show that N-cadherin-dependent adhesion promotes the migration of mouse MGE cells in vitro. Conversely, mouse MGE cells electroporated with a construct interfering with cadherin function show reduced cell motility, leading process instability, and impaired polarization associated with abnormal myosin IIB dynamics. In vivo, the capability of electroporated MGE cells to invade the developing cortical plate is altered. Using genetic ablation of N-cadherin in mouse embryos, we show that N-cadherin-depleted MGEs are severely disorganized. MGE cells hardly exit the disorganized proliferative area. N-cadherin ablation at the postmitotic stage, which does not affect MGE morphogenesis, alters MGE cell motility and directionality. The tangential migration to the cortex of N-cadherin ablated MGE cells is delayed, and their radial migration within the cortical plate is perturbed. Altogether, these results identify N-cadherin as a pivotal adhesion substrate that activates cell motility in future cortical interneurons and maintains cell polarity over their long-distance migration to the developing cortex.

XLMR protein related to neurite extension (Xpn/KIAA2022) regulates cell-cell and cell-matrix adhesion and migration

X-linked mental retardation (XLMR) is a common cause of moderate to severe intellectual disability in males. XLMR protein related to neurite extension (Xpn, also known as KIAA2022) has been implicated as a gene responsible for XLMR in humans. Although Xpn is highly expressed in the developing brain and is involved in neurite outgrowth in PC12 cells and neurons, little is known about the functional role of Xpn. Here, we show that Xpn regulates cell-cell and cell-matrix adhesion and migration in PC12 cells. Xpn knockdown enhanced cell-cell and cell-matrix adhesion mediated by N-cadherin and β1-integrin, respectively. N-Cadherin and β1-integrin expression at the mRNA and protein levels was significantly increased in Xpn knockdown PC12 cells. Furthermore, overexpressed Xpn protein was strongly expressed in the nuclei of PC12 and 293T cells. Finally, depletion of Xpn perturbed cellular migration by enhancing N-cadherin and β1-integrin expression in a PC12 cell wound healing assay. We conclude that Xpn regulates cell-cell and cell-matrix adhesion and cellular migration by regulating the expression of adhesion molecules.

olig2-Expressing hindbrain cells are required for migrating facial motor neurons

The complicated trajectory of facial motor neuron migration requires coordination of intrinsic signals and cues from the surrounding environment. Migration begins in rhombomere (r) 4 where facial motor neurons are born and proceeds in a caudal direction. Once facial motor neurons reach their target rhombomeres, they migrate laterally and radially from the ventral neural tube. In zebrafish, as facial motor neurons migrate through r5/r6, they pass near cells that express olig2, which encodes a bHLH transcription factor. In this study, we found that olig2 function is required for facial motor neurons to complete their caudal migration into r6 and r7 and form stereotypical clusters. Additionally, embryos that lack mafba function, in which facial motor neurons also fail to complete caudal migration, lack olig2 expression in r5 and r6. Our data raise the possibility that cells expressing olig2 are intermediate targets that help guide facial motor neuron migration.

Cadherins in brain morphogenesis and wiring

Cadherins are Ca(2+)-dependent cell-cell adhesion molecules that play critical roles in animal morphogenesis. Various cadherin-related molecules have also been identified, which show diverse functions, not only for the regulation of cell adhesion but also for that of cell proliferation and planar cell polarity. During the past decade, understanding of the roles of these molecules in the nervous system has significantly progressed. They are important not only for the development of the nervous system but also for its functions and, in turn, for neural disorders. In this review, we discuss the roles of cadherins and related molecules in neural development and function in the vertebrate brain.

Cadherins and neuropsychiatric disorders

Cadherins mediate cell-cell adhesion but are also involved in intracellular signaling pathways associated with neuropsychiatric disease. Most of the ∼100 cadherins that are expressed in the brain exhibit characteristic spatiotemporal expression profiles. Cadherins have been shown to regulate neural tube regionalization, neuronal migration, gray matter differentiation, neural circuit formation, spine morphology, synapse formation and synaptic remodeling. The dysfunction of the cadherin-based adhesive system may alter functional connectivity and coherent information processing in the human brain in neuropsychiatric disease. Several neuropsychiatric disorders, such as epilepsy/mental retardation, autism, bipolar disease and schizophrenia, have been associated with cadherins, mostly by genome-wide association studies. For example, CDH15 and PCDH19 are associated with cognitive impairment; CDH5, CDH8, CDH9, CDH10, CDH13, CDH15, PCDH10, PCDH19 and PCDHb4 with autism; CDH7, CDH12, CDH18, PCDH12 and FAT with bipolar disease and schizophrenia; and CDH11, CDH12 and CDH13 with methamphetamine and alcohol dependency. To date, disease-causing mutations are established for PCDH19 in patients with epilepsy, cognitive impairment and/or autistic features. In conclusion, genes encoding members of the cadherin superfamily are of special interest in the pathogenesis of neuropsychiatric disease because cadherins play a pivotal role in the development of the neural circuitry as well as in mature synaptic function.

Re-expression of N-cadherin in remyelinating lesions of experimental inflammatory demyelination

The cell adhesion molecule N-cadherin is involved in several processes during central nervous system development, but also in certain pathologic conditions in the adult brain, including tumorigenesis and Alzheimer's disease. N-cadherin function in inflammatory demyelinating disease has so far not been investigated. In vitro studies suggest a role of N-cadherin in myelination; on the other hand N-cadherin has been implicated in the formation of the glial scar, which is believed to impede remyelination. The aim of our study was to investigate the expression pattern of N-cadherin immunoreactivity in experimental autoimmune encephalomyelitis induced by myelin oligodendrocyte glycoprotein (MOG-EAE), an animal model closely mimicking multiple sclerosis. It allows a detailed evaluation of all stages of de- and remyelination during lesion development. Immunopathological evaluation was performed on paraffin-embedded CNS sections sampled at days 20 to 120 post immunization. We found a predominant expression of N-cadherin on oligodendrocytes in early remyelinating lesions, while in fully remyelinated shadow plaques there was no detectable immunoreactivity for N-cadherin. This expression pattern indicates a role of N-cadherin in the initiation of remyelination, most likely by providing a guidance between myelin lamellae and oligodendrocytes. Once the initial contact is made N-cadherin is then rapidly downregulated and virtually absent after completion of the repair process, analog to its known role in developmental myelination. Our results show that N-cadherin plays an important role in creating a remyelination-facilitating environment.

Molecular regionalization of the diencephalon

The anatomic complexity of the diencephalon depends on precise molecular and cellular regulative mechanisms orchestrated by regional morphogenetic organizers at the neural tube stage. In the diencephalon, like in other neural tube regions, dorsal and ventral signals codify positional information to specify ventro-dorsal regionalization. Retinoic acid, Fgf8, BMPs, and Wnts signals are the molecular factors acting upon the diencephalic epithelium to specify dorsal structures, while Shh is the main ventralizing signal. A central diencephalic organizer, the zona limitans intrathalamica (ZLI), appears after neurulation in the central diencephalic alar plate, establishing additional antero-posterior positional information inside diencephalic alar plate. Based on Shh expression, the ZLI acts as a morphogenetic center, which cooperates with other signals in thalamic specification and pattering in the alar plate of diencephalon. Indeed, Shh is expressed first in the basal plate extending dorsally through the ZLI epithelium as the development proceeds. Despite the importance of ZLI in diencephalic morphogenesis the mechanisms that regulate its development remain incompletely understood. Actually, controversial interpretations in different experimental models have been proposed. That is, experimental results have suggested that (i) the juxtaposition of the molecularly heterogeneous neuroepithelial areas, (ii) cell reorganization in the epithelium, and/or (iii) planar and vertical inductions in the neural epithelium, are required for ZLI specification and development. We will review some experimental data to approach the study of the molecular regulation of diencephalic regionalization, with special interest in the cellular mechanisms underlying planar inductions.

Unit operations of tissue development: epithelial folding

The development of multicellular organisms relies on a small set of construction techniques-assembly, sculpting, and folding-that are spatially and temporally regulated in a combinatorial manner to produce the diversity of tissues within the body. These basic processes are well conserved across tissue types and species at the level of both genes and mechanisms. Here we review the signaling, patterning, and biomechanical transformations that occur in two well-studied model systems of epithelial folding to illustrate both the complexity and modularity of tissue development. In particular, we discuss the possibility of a spatial code specifying morphogenesis. To decipher this code, engineers and scientists need to establish quantitative experimental systems and to develop models that address mechanisms at multiple levels of organization, from gene sequence to tissue biomechanics. In turn, quantitative models of embryogenesis can inspire novel methods for creating synthetic organs and treating degenerative tissue diseases.

Patterning and compartment formation in the diencephalon

The diencephalon gives rise to structures that play an important role in connecting the anterior forebrain with the rest of the central nervous system. The thalamus is the major diencephalic derivative that functions as a relay station between the cortex and other lower order sensory systems. Almost two decades ago, neuromeric/prosomeric models were proposed describing the subdivision and potential segmentation of the diencephalon. Unlike the laminar structure of the cortex, the diencephalon is progressively divided into distinct functional compartments consisting principally of thalamus, epithalamus, pretectum, and hypothalamus. Neurons generated within these domains further aggregate to form clusters called nuclei, which form specific structural and functional units. We review the recent advances in understanding the genetic mechanisms that are involved in the patterning and compartment formation of the diencephalon.

N-cadherin regulates primary motor axon growth and branching during zebrafish embryonic development

N-cadherin is a classical type I cadherin that contributes to the formation of neural circuits by regulating growth cone migration and the formation of synaptic contacts. This study analyzed the role of N-cadherin in primary motor axons growth during development of the zebrafish (Danio rerio) embryo. After exiting the spinal cord, primary motor axons migrate ventrally through a common pathway and form the first neuromuscular junction with the muscle pioneer cells located at the horizontal myoseptum, which serves as a choice point for cell-type-specific pathway selection. Analysis of N-cadherin mutants (cdh2(hi3644Tg) ) and embryos injected with N-cadherin antisense morpholinos showed primary motor axons extending aberrant axonal branches at the choice point in ∼40% of the somitic hemisegments and an ∼150% increase in the number of branches per axon length within the ventral myotome. Analysis of individual axons trajectories showed that the caudal (CaP) and rostral (RoP) motor neurons axons formed aberrant branches at the choice point that abnormally extended in the rostrocaudal axis and ventrally to the horizontal myoseptum. Expression of a dominant-interfering N-cadherin cytoplasmic domain in primary motor neurons caused some axons to stall abnormally at the horizontal myoseptum and to impair their migration into the ventral myotome. However, in N-cadherin-depleted embryos, the majority of primary motor axons innervated their appropriate myotomal territories, indicating that N-cadherin regulates motor axon growth and branching without severely affecting the mechanisms that control axonal target selection.