Neuronal circuits are subdivided by differential expression of type-II classic cadherins in postnatal mouse brains

Cdh11: Roles in different diseases and potential value in disease diagnosis and treatment

Cadherin is a homophilic, Ca2+-dependent cell adhesion glycoprotein that mediates cell-cell adhesion. Among them, Cadherin-11 (CDH11), as a classical cadherin, participates in and influences many crucial aspects of human growth and development. Furthermore, The involvement of CDH11 has been identified in an increasing number of diseases, primarily including various tumorous diseases, fibrotic diseases, autoimmune diseases, neurodevelopmental disorders, and more. In various tumorous diseases, CDH11 acts not only as a tumor suppressor but can also promote migration and invasion of certain tumors through various mechanisms. Likewise, in non-tumorous diseases, CDH11 remains a pivotal factor in disease progression. In this context, we summarize the specific functionalities and mechanisms of CDH11 in various diseases, aiming to gain a more comprehensive understanding of the potential value of CDH11 in disease diagnosis and treatment. This endeavor seeks to provide more effective diagnostic and therapeutic strategies for clinical management across diverse diseases.

Development and cadherin-mediated control of prefrontal corticostriatal projections in mice

Action-outcome associations depend on prefrontal cortex (PFC) projections to the dorsal striatum. To assess how these projections form, we measured PFC axon patterning, synapse formation, and functional maturation in the postnatally developing mouse striatum. Using Hotspot analysis, we show that PFC axons form an adult-like pattern of clustered terminations in the first postnatal week that remains largely stable thereafter. PFC-striatal synaptic strength is adult-like by P21, while excitatory synapse density increases until adulthood. We then tested how the targeted deletion of a candidate adhesion/guidance protein, Cadherin-8 (Cdh8), from corticostriatal neurons regulates pathway development. Mutant mice showed diminished PFC axon targeting and reduced spontaneous glutamatergic synaptic activity in the dorsal striatum. They also exhibited impaired behavioral performance in action-outcome learning. The data show that PFC-striatal axons form striatal territories through an early, directed growth model and they highlight essential contributions of Cdh8 to the anatomical and functional features critical for the formation of action-outcome associations.

Development of prefrontal corticostriatal connectivity in mice

Rational decision making is grounded in learning to associate actions with outcomes, a process that depends on projections from prefrontal cortex to dorsomedial striatum. Symptoms associated with a variety of human pathological conditions ranging from schizophrenia and autism to Huntington's and Parkinson's disease point toward functional deficits in this projection, but its development is not well understood, making it difficult to investigate how perturbations in development of this circuitry could contribute to pathophysiology. We applied a novel strategy based on Hotspot Analysis to assess the developmental progression of anatomical positioning of prefrontal cortex to striatal projections. Corticostriatal axonal territories established at P7 expand in concert with striatal growth but remain largely unchanged in positioning through adulthood, indicating they are generated by directed, targeted growth and not modified extensively by postnatal experience. Consistent with these findings, corticostriatal synaptogenesis increased steadily from P7 to P56, with no evidence for widescale pruning. As corticostriatal synapse density increased over late postnatal ages, the strength of evoked PFC input onto dorsomedial striatal projection neurons also increased, but spontaneous glutamatergic synaptic activity was stable. Based on its pattern of expression, we asked whether the adhesion protein, Cdh8, influenced this progression. In mice lacking Cdh8 in PFC corticostriatal projection neurons, axon terminal fields in dorsal striatum shifted ventrally. Corticostriatal synaptogenesis was unimpeded, but spontaneous EPSC frequency declined and mice failed to learn to associate an action with an outcome. Collectively these findings show that corticostriatal axons grow to their target zone and are restrained from an early age, do not undergo postnatal synapse pruning as the most dominant models predict, and that a relatively modest shift in terminal arbor positioning and synapse function has an outsized, negative impact on corticostriatal-dependent behavior.

Catenin signaling controls phrenic motor neuron development and function during a narrow temporal window

Phrenic Motor Column (PMC) neurons are a specialized subset of motor neurons (MNs) that provide the only motor innervation to the diaphragm muscle and are therefore essential for survival. Despite their critical role, the mechanisms that control phrenic MN development and function are not well understood. Here, we show that catenin-mediated cadherin adhesive function is required for multiple aspects of phrenic MN development. Deletion of β- and γ-catenin from MN progenitors results in perinatal lethality and a severe reduction in phrenic MN bursting activity. In the absence of catenin signaling, phrenic MN topography is eroded, MN clustering is lost and phrenic axons and dendrites fail to grow appropriately. Despite the essential requirement for catenins in early phrenic MN development, they appear to be dispensable for phrenic MN maintenance, as catenin deletion from postmitotic MNs does not impact phrenic MN topography or function. Our data reveal a fundamental role for catenins in PMC development and suggest that distinct mechanisms are likely to control PMC maintenance.

Catenin signaling controls phrenic motor neuron development and function during a narrow temporal window

Phrenic Motor Column (PMC) neurons are a specialized subset of motor neurons (MNs) that provide the only motor innervation to the diaphragm muscle and are therefore essential for survival. Despite their critical role, the mechanisms that control phrenic MN development and function are not well understood. Here, we show that catenin-mediated cadherin adhesive function is required for multiple aspects of phrenic MN development. Deletion of β - and γ -catenin from MN progenitors results in perinatal lethality and a severe reduction in phrenic MN bursting activity. In the absence of catenin signaling, phrenic MN topography is eroded, MN clustering is lost and phrenic axons and dendrites fail to grow appropriately. Despite the essential requirement for catenins in early phrenic MN development, they appear to be dispensable for phrenic MN maintenance, as catenin deletion from postmitotic MNs does not impact phrenic MN topography or function. Our data reveal a fundamental role for catenins in PMC development and suggest that distinct mechanisms are likely to control PMC maintenance.

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.

Deficiency of protocadherin 9 leads to reduction in positive emotional behaviour

Protocadherin 9 (Pcdh9) is a member of the cadherin superfamily and is uniquely expressed in the vestibular and limbic systems; however, its physiological role remains unclear. Here, we studied the expression of Pcdh9 in the limbic system and phenotypes of Pcdh9-knock-out mice (Pcdh9 KO mice). Pcdh9 mRNA was expressed in the fear extinction neurons that express protein phosphatase 1 regulatory subunit 1 B (Ppp1r1b) in the posterior part of the basolateral amygdala (pBLA), as well as in the Cornu Ammonis (CA) and Dentate Gyrus (DG) neurons of the hippocampus. We show that the Pcdh9 protein was often localised at synapses. Phenotypic analysis of Pcdh9 KO mice revealed no apparent morphological abnormalities in the pBLA but a decrease in the spine number of CA neurons. Further, the Pcdh9 KO mice were related to features such as the abnormal optokinetic response, less approach to novel objects, and reduced fear extinction during recovery from the fear. These results suggest that Pcdh9 is involved in eliciting positive emotional behaviours, possibly via fear extinction neurons in the pBLA and/or synaptic activity in the hippocampal neurons, and normal optokinetic eye movement in brainstem optokinetic system-related neurons.

Epithelial to mesenchymal transition during mammalian neural crest cell delamination

Epithelial to mesenchymal transition (EMT) is a well-defined cellular process that was discovered in chicken embryos and described as "epithelial to mesenchymal transformation" [1]. During EMT, epithelial cells lose their epithelial features and acquire mesenchymal character with migratory potential. EMT has subsequently been shown to be essential for both developmental and pathological processes including embryo morphogenesis, wound healing, tissue fibrosis and cancer [2]. During the past 5 years, interest and study of EMT especially in cancer biology have increased exponentially due to the implied role of EMT in multiple aspects of malignancy such as cell invasion, survival, stemness, metastasis, therapeutic resistance and tumor heterogeneity [3]. Since the process of EMT in embryogenesis and cancer progression shares similar phenotypic changes, core transcription factors and molecular mechanisms, it has been proposed that the initiation and development of carcinoma could be attributed to abnormal activation of EMT factors usually required for normal embryo development. Therefore, developmental EMT mechanisms, whose timing, location, and tissue origin are strictly regulated, could prove useful for uncovering new insights into the phenotypic changes and corresponding gene regulatory control of EMT under pathological conditions. In this review, we initially provide an overview of the phenotypic and molecular mechanisms involved in EMT and discuss the newly emerging concept of epithelial to mesenchymal plasticity (EMP). Then we focus on our current knowledge of a classic developmental EMT event, neural crest cell (NCC) delamination, highlighting key differences in our understanding of NCC EMT between mammalian and non-mammalian species. Lastly, we highlight available tools and future directions to advance our understanding of mammalian NCC EMT.

The distribution of Cdh20 mRNA demarcates somatotopic subregions and subpopulations of spiny projection neurons in the rat dorsolateral striatum

The dorsolateral striatum (DLS) of rodents is functionally subdivided into somatotopic subregions that represent each body part along both the dorsoventral and anteroposterior (A-P) axes and play crucial roles in sensorimotor functions via corticostriatal pathways. However, little is known about the spatial gene expression patterns and heterogeneity of spiny projection neurons (SPNs) within somatotopic subregions. Here, we show that the cell adhesion molecule gene Cdh20, which encodes a Type II cadherin, is expressed in discrete subregions covering the inner orofacial area and part of the forelimb area in the ventral domain of the DLS (v-DLS) in rats. Cdh20-expressing cells were localized in the v-DLS at the intermediate level of the striatum along the A-P axis and could be classified as direct-pathway SPNs or indirect-pathway SPNs. Unexpectedly, comprehensive analysis revealed that Cdh20 is expressed in SPNs in the rat DLS but not in the mouse DLS or the ferret putamen (Pu). Our observations reveal that Cdh20 expression demarcates somatotopic subregions and subpopulations of SPNs specifically in the rat DLS and suggest divergent regulation of genes differentially expressed in the v-DLS and Pu among mammals.

Regulation of Neural Circuit Development by Cadherin-11 Provides Implications for Autism

Autism spectrum disorder (ASD) is a neurologic condition characterized by alterations in social interaction and communication, and restricted and/or repetitive behaviors. The classical Type II cadherins cadherin-8 (Cdh8, CDH8) and cadherin-11 (Cdh11, CDH11) have been implicated as autism risk gene candidates. To explore the role of cadherins in the etiology of autism, we investigated their expression patterns during mouse brain development and in autism-specific human tissue. In mice, expression of cadherin-8 and cadherin-11 was developmentally regulated and enriched in the cortex, hippocampus, and thalamus/striatum during the peak of dendrite formation and synaptogenesis. Both cadherins were expressed in synaptic compartments but only cadherin-8 associated with the excitatory synaptic marker neuroligin-1. Induced pluripotent stem cell (iPSC)-derived cortical neural precursor cells (NPCs) and cortical organoids generated from individuals with autism showed upregulated CDH8 expression levels, but downregulated CDH11. We used Cdh11 knock-out (KO) mice of both sexes to analyze the function of cadherin-11, which could help explain phenotypes observed in autism. Cdh11-/- hippocampal neurons exhibited increased dendritic complexity along with altered neuronal and synaptic activity. Similar to the expression profiles in human tissue, levels of cadherin-8 were significantly elevated in Cdh11 KO brains. Additionally, excitatory synaptic markers neuroligin-1 and postsynaptic density (PSD)-95 were both increased. Together, these results strongly suggest that cadherin-11 is involved in regulating the development of neuronal circuitry and that alterations in the expression levels of cadherin-11 may contribute to the etiology of autism.

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.

New Optical Tools to Study Neural Circuit Assembly in the Retina

During development, neurons navigate a tangled thicket of thousands of axons and dendrites to synapse with just a few specific targets. This phenomenon termed wiring specificity, is critical to the assembly of neural circuits and the way neurons manage this feat is only now becoming clear. Recent studies in the mouse retina are shedding new insight into this process. They show that specific wiring arises through a series of stages that include: directed axonal and dendritic growth, the formation of neuropil layers, positioning of such layers, and matching of co-laminar synaptic partners. Each stage appears to be directed by a distinct family of recognition molecules, suggesting that the combinatorial expression of such family members might act as a blueprint for retinal connectivity. By reviewing the evidence in support of each stage, and by considering their underlying molecular mechanisms, we attempt to synthesize these results into a wiring model which generates testable predictions for future studies. Finally, we conclude by highlighting new optical methods that could be used to address such predictions and gain further insight into this fundamental process.

Segregated expressions of autism risk genes Cdh11 and Cdh9 in autism-relevant regions of developing cerebellum

Results of recent genome-wide association studies (GWAS) and whole genome sequencing (WGS) highlighted type II cadherins as risk genes for autism spectrum disorders (ASD). To determine whether these cadherins may be linked to the morphogenesis of ASD-relevant brain regions, in situ hybridization (ISH) experiments were carried out to examine the mRNA expression profiles of two ASD-associated cadherins, Cdh9 and Cdh11, in the developing cerebellum. During the first postnatal week, both Cdh9 and Cdh11 were expressed at high levels in segregated sub-populations of Purkinje cells in the cerebellum, and the expression of both genes was declined as development proceeded. Developmental expression of Cdh11 was largely confined to dorsal lobules (lobules VI/VII) of the vermis as well as the lateral hemisphere area equivalent to the Crus I and Crus II areas in human brains, areas known to mediate high order cognitive functions in adults. Moreover, in lobules VI/VII of the vermis, Cdh9 and Cdh11 were expressed in a complementary pattern with the Cdh11-expressing areas flanked by Cdh9-expressing areas. Interestingly, the high level of Cdh11 expression in the central domain of lobules VI/VII was correlated with a low level of expression of the Purkinje cell marker calbindin, coinciding with a delayed maturation of Purkinje cells in the same area. These findings suggest that these two ASD-associated cadherins may exert distinct but coordinated functions to regulate the wiring of ASD-relevant circuits in the cerebellum.

Design Principles for Pluripotent Stem Cell-Derived Organoid Engineering

Human morphogenesis is a complex process involving distinct microenvironmental and physical signals that are manipulated in space and time to give rise to complex tissues and organs. Advances in pluripotent stem cell (PSC) technology have promoted the in vitro recreation of processes involved in human morphogenesis. The development of organoids from human PSCs represents one reliable source for modeling a large spectrum of human disorders, as well as a promising approach for drug screening and toxicological tests. Based on the "self-organization" capacity of stem cells, different PSC-derived organoids have been created; however, considerable differences between in vitro-generated PSC-derived organoids and their in vivo counterparts have been reported. Advances in the bioengineering field have allowed the manipulation of different components, including cellular and noncellular factors, to better mimic the in vivo microenvironment. In this review, we focus on different examples of bioengineering approaches used to promote the self-organization of stem cells, including assembly, patterning, and morphogenesis in vitro, contributing to tissue-like structure formation.

Precision in the development of neocortical architecture: From progenitors to cortical networks

Of all brain regions, the 6-layered neocortex has undergone the most dramatic changes in size and complexity during mammalian brain evolution. These changes, occurring in the context of a conserved set of organizational features that emerge through stereotypical developmental processes, are considered responsible for the cognitive capacities and sensory specializations represented within the mammalian clade. The modern experimental era of developmental neurobiology, spanning 6 decades, has deciphered a number of mechanisms responsible for producing the diversity of cortical neuron types, their precise connectivity and the role of gene by environment interactions. Here, experiments providing insight into the development of cortical projection neuron differentiation and connectivity are reviewed. This current perspective integrates discussion of classic studies and new findings, based on recent technical advances, to highlight an improved understanding of the neuronal complexity and precise connectivity of cortical circuitry. These descriptive advances bring new opportunities for studies related to the developmental origins of cortical circuits that will, in turn, improve the prospects of identifying pathogenic targets of neurodevelopmental disorders.

The adhesion molecule cadherin 11 is essential for acquisition of normal hearing ability through middle ear development in the mouse

Cadherin 11 (Cdh11), a member of the cadherin adhesion molecule family, is expressed in various regions of the brain as well as the head and ear. To gain further insights into the roles of Cdh11 in the development of the ear, we performed behavioral tests using Cdh11 knockout (KO) mice. KO mice showed reduced acoustic startle responses and increased thresholds for auditory brainstem responses, indicating moderate hearing loss. The auditory bulla volume and ratio of air-filled to non-air-filled space in the middle ear cavity were reduced in KO mice, potentially causing conductive hearing loss. Furthermore, residual mesenchymal and inflammatory cells were observed in the middle ear cavity of KO mice. Cdh11 was expressed in developing mesenchymal cells just before the start of cavitation, indicating that Cdh11 may be directly involved in middle ear cavitation. Since the auditory bulla is derived from the neural crest, the regulation of neural crest-derived cells by Cdh11 may be responsible for structural development. This mutant mouse may be a promising animal model for elucidating the causes of conductive hearing loss and otitis media.

Zic4-Lineage Cells Increase Their Contribution to Visual Thalamic Nuclei during Murine Embryogenesis If They Are Homozygous or Heterozygous for Loss of Pax6 Function

Our aim was to study the mechanisms that contribute to the development of discrete thalamic nuclei during mouse embryogenesis (both sexes included). We characterized the expression of the transcription factor coding gene Zic4 and the distribution of cells that expressed Zic4 in their lineage. We used genetic fate mapping to show that Zic4-lineage cells mainly contribute to a subset of thalamic nuclei, in particular the lateral geniculate nuclei (LGNs), which are crucial components of the visual pathway. We observed that almost all Zic4-lineage diencephalic progenitors express the transcription factor Pax6 at variable location-dependent levels. We used conditional mutagenesis to delete either one or both copies of Pax6 from Zic4-lineage cells. We found that Zic4-lineage cells carrying either homozygous or heterozygous loss of Pax6 contributed in abnormally high numbers to one or both of the main lateral geniculate nuclei (LGNs). This could not be attributed to a change in cell production and was likely due to altered sorting of thalamic cells. Our results indicate that positional information encoded by the levels of Pax6 in diencephalic progenitors is an important determinant of the eventual locations of their daughter cells.

A novel mutation in CDH11, encoding cadherin-11, cause Branchioskeletogenital (Elsahy-Waters) syndrome

Cadherins are cell-adhesion molecules that control morphogenesis, cell migration, and cell shape changes during multiple developmental processes. Until now four distinct cadherins have been implicated in human Mendelian disorders, mainly featuring skin, retinal and hearing manifestations. Branchio-skeleto-genital (or Elsahy-Waters) syndrome (BSGS) is an ultra-rare condition featuring a characteristic face, premature loss of teeth, vertebral and genital anomalies, and intellectual disability. We have studied two sibs with BSGS originally described by Castori et al. in 2010. Exome sequencing led to the identification of a novel homozygous nonsense variant in the first exon of the cadherin-11 gene (CDH11), which results in a prematurely truncated form of the protein. Recessive variants in CDH11 have been recently demonstrated in two other sporadic patients and a pair of sisters affected by BSGS. Although the function of this cadherin (also termed Osteoblast-Cadherin) is not completely understood, its prevalent expression in osteoblastic cell lines and up-regulation during differentiation suggest a specific function in bone formation and development. This study identifies a novel loss-of-function variant in CDH11 as a cause of BSGS and supports the role of cadherin-11 as a key player in axial and craniofacial malformations.

Organizing activity of Fgf8 on the anterior telencephalon

The anterior part of the embryonic telencephalon gives rise to several brain regions that are important for animal behavior, including the frontal cortex (FC) and the olfactory bulb. The FC plays an important role in decision-making behaviors, such as social and cognitive behavior, and the olfactory bulb is involved in olfaction. Here, we show the organizing activity of fibroblast growth factor 8 (Fgf8) in the regionalization of the anterior telencephalon, specifically the FC and the olfactory bulb. Misexpression of Fgf8 in the most anterior part of the mouse telencephalon at embryonic day 11.5 (E11.5) by ex utero electroporation resulted in a lateral shift of dorsal FC subdivision markers and a lateral expansion of the dorsomedial part of the FC, the future anterior cingulate and prelimbic cortex. Fgf8-transfected brains had lacked ventral FC, including the future orbital cortex, which was replaced by the expanded olfactory bulb. The olfactory region occupied a larger area of the FC when transfection efficiency of Fgf8 was higher. These results suggest that Fgf8 regulates the proportions of the FC and olfactory bulb in the anterior telencephalon and has a medializing effect on the formation of FC subdivisions.

Cadherin-10 Maintains Excitatory/Inhibitory Ratio through Interactions with Synaptic Proteins

Appropriate excitatory/inhibitory (E/I) balance is essential for normal cortical function and is altered in some psychiatric disorders, including autism spectrum disorders (ASDs). Cell-autonomous molecular mechanisms that control the balance of excitatory and inhibitory synapse function remain poorly understood; no proteins that regulate excitatory and inhibitory synapse strength in a coordinated reciprocal manner have been identified. Using super-resolution imaging, electrophysiology, and molecular manipulations, we show that cadherin-10, encoded by CDH10 within the ASD risk locus 5p14.1, maintains both excitatory and inhibitory synaptic scaffold structure in cultured cortical neurons from rats of both sexes. Cadherin-10 localizes to both excitatory and inhibitory synapses in neocortex, where it is organized into nanoscale puncta that influence the size of their associated PSDs. Knockdown of cadherin-10 reduces excitatory but increases inhibitory synapse size and strength, altering the E/I ratio in cortical neurons. Furthermore, cadherin-10 exhibits differential participation in complexes with PSD-95 and gephyrin, which may underlie its role in maintaining the E/I ratio. Our data provide a new mechanism whereby a protein encoded by a common ASD risk factor controls E/I ratios by regulating excitatory and inhibitory synapses in opposing directions.SIGNIFICANCE STATEMENT The correct balance between excitatory/inhibitory (E/I) is crucial for normal brain function and is altered in psychiatric disorders such as autism. However, the molecular mechanisms that underlie this balance remain elusive. To address this, we studied cadherin-10, an adhesion protein that is genetically linked to autism and understudied at the cellular level. Using a combination of advanced microscopy techniques and electrophysiology, we show that cadherin-10 forms nanoscale puncta at excitatory and inhibitory synapses, maintains excitatory and inhibitory synaptic structure, and is essential for maintaining the correct balance between excitation and inhibition in neuronal dendrites. These findings reveal a new mechanism by which E/I balance is controlled in neurons and may bear relevance to synaptic dysfunction in autism.

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.

Regional Cellular Environment Shapes Phenotypic Variations of Hippocampal and Neocortical Chandelier Cells

Different cortical regions processing distinct information, such as the hippocampus and the neocortex, share common cellular components and circuit motifs but form unique networks by modifying these cardinal units. Cortical circuits include diverse types of GABAergic interneurons (INs) that shape activity of excitatory principal neurons (PNs). Canonical IN types conserved across distinct cortical regions have been defined by their morphological, electrophysiological, and neurochemical properties. However, it remains largely unknown whether canonical IN types undergo specific modifications in distinct cortical regions and display "regional variants." It is also poorly understood whether such phenotypic variations are shaped by early specification or regional cellular environment. The chandelier cell (ChC) is a highly stereotyped IN type that innervates axon initial segments of PNs and thus serves as a good model with which to address this issue. Here, we show that Cadherin-6 (Cdh6), a homophilic cell adhesion molecule, is a reliable marker of ChCs and Cdh6-CreER mice (both sexes) provide genetic access to hippocampal ChCs (h-ChCs). We demonstrate that, compared with neocortical ChCs (nc-ChCs), h-ChCs cover twice as much area and innervate twice as many PNs. Interestingly, a subclass of h-ChCs exhibits calretinin (CR) expression, which is not found in nc-ChCs. Furthermore, we find that h-ChCs appear to be born earlier than nc-ChCs. Surprisingly, despite the difference in temporal origins, ChCs display host-region-dependent axonal/synaptic organization and CR expression when transplanted heterotopically. These results suggest that local cellular environment plays a critical role in shaping terminal phenotypes of regional IN variants in the hippocampus and the neocortex.SIGNIFICANCE STATEMENT Canonical interneuron (IN) types conserved across distinct cortical regions such as the hippocampus and the neocortex are defined by morphology, physiology, and gene expression. However, it remains unknown whether they display phenotypic variations in different cortical regions. In addition, it is unclear whether terminal phenotypes of regional IN variants belonging to a canonical IN type are determined intrinsically or extrinsically. Our results provide evidence of striking differences in axonal/synaptic organization and calretinin expression between hippocampal chandelier cells (ChCs) and neocortical ChCs. They also reveal that local cellular environment in distinct cortical regions regulates these terminal phenotypes. Therefore, our study suggests that local cortical environment shapes the phenotypes of regional IN variants, which may be required for unique circuit operations in distinct cortical regions.

Loss of X-linked Protocadherin-19 differentially affects the behavior of heterozygous female and hemizygous male mice

Mutations in the X-linked gene Protocadherin-19 (Pcdh19) cause female-limited epilepsy and mental retardation in humans. Although Pcdh19 is known to be a homophilic cell-cell adhesion molecule, how its mutations bring about female-specific disorders remains elusive. Here, we report the effects of Pcdh19 knockout in mice on their development and behavior. Pcdh19 was expressed in various brain regions including the cerebral cortex and hippocampus. Although Pcdh19-positive cells were evenly distributed in layer V of wild-type cortices, their distribution became a mosaic in Pcdh19 heterozygous female cortices. In cortical and hippocampal neurons, Pcdh19 was localized along their dendrites, showing occasional accumulation on synapses. Pcdh19 mutants, however, displayed no detectable abnormalities in dendrite and spine morphology of layer V neurons. Nevertheless, Pcdh19 hemizygous males and heterozygous females showed impaired behaviors including activity defects under stress conditions. Notably, only heterozygous females exhibited decreased fear responses. In addition, Pcdh19 overexpression in wild-type cortices led to ectopic clustering of Pcdh19-positive neurons. These results suggest that Pcdh19 is required for behavioral control in mice, but its genetic loss differentially affects the male and female behavior, as seen in human, and they also support the hypothesis that the mosaic expression of Pcdh19 in brains perturbs neuronal interactions.

A dual-strategy expression screen for candidate connectivity labels in the developing thalamus

The thalamus or “inner chamber” of the brain is divided into ~30 discrete nuclei, with highly specific patterns of afferent and efferent connectivity. To identify genes that may direct these patterns of connectivity, we used two strategies. First, we used a bioinformatics pipeline to survey the predicted proteomes of nematode, fruitfly, mouse and human for extracellular proteins containing any of a list of motifs found in known guidance or connectivity molecules. Second, we performed clustering analyses on the Allen Developing Mouse Brain Atlas data to identify genes encoding surface proteins expressed with temporal profiles similar to known guidance or connectivity molecules. In both cases, we then screened the resultant genes for selective expression patterns in the developing thalamus. These approaches identified 82 candidate connectivity labels in the developing thalamus. These molecules include many members of the Ephrin, Eph-receptor, cadherin, protocadherin, semaphorin, plexin, Odz/teneurin, Neto, cerebellin, calsyntenin and Netrin-G families, as well as diverse members of the immunoglobulin (Ig) and leucine-rich receptor (LRR) superfamilies, receptor tyrosine kinases and phosphatases, a variety of growth factors and receptors, and a large number of miscellaneous membrane-associated or secreted proteins not previously implicated in axonal guidance or neuronal connectivity. The diversity of their expression patterns indicates that thalamic nuclei are highly differentiated from each other, with each one displaying a unique repertoire of these molecules, consistent with a combinatorial logic to the specification of thalamic connectivity.

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.

Complex and dynamic expression of cadherins in the embryonic marmoset cerebral cortex

Cadherin is a cell adhesion molecule widely expressed in the nervous system. Previously, we analyzed the expression of nine classic cadherins (Cdh4, Cdh6, Cdh7, Cdh8, Cdh9, Cdh10, Cdh11, Cdh12, and Cdh20) and T-cadherin (Cdh13) in the developing postnatal common marmoset (Callithrix jacchus) brain, and found differential expressions between mice and marmosets. In this study, to explore primate-specific cadherin expression at the embryonic stage, we extensively analyzed the expression of these cadherins in the developing embryonic marmoset brain. Each cadherin showed differential spatial and temporal expression and exhibited temporally complicated expression. Furthermore, the expression of some cadherins differed from that in rodent brains, even at the embryonic stage. These results suggest the possibility that the differential expressions of diverse cadherins are involved in primate specific cortical development, from the prenatal to postnatal period.

The effects of lifelong blindness on murine neuroanatomy and gene expression

Mammalian neocortical development is regulated by neural patterning mechanisms, with distinct sensory and motor areas arising through the process of arealization. This development occurs alongside developing central or peripheral sensory systems. Specifically, the parcellation of neocortex into specific areas of distinct cytoarchitecture, connectivity and function during development is reliant upon both cortically intrinsic mechanisms, such as gene expression, and extrinsic processes, such as input from the sensory receptors. This developmental program shifts from patterning to maintenance as the animal ages and is believed to be active throughout life, where the brain’s organization is stable yet plastic. In this study, we characterize the long-term effects of early removal of visual input via bilateral enucleation at birth. To understand the long-term effects of early blindness we conducted anatomical and molecular assays 18 months after enucleation, near the end of lifespan in the mouse. Bilateral enucleation early in life leads to long-term, stable size reductions of the thalamic lateral geniculate nucleus (LGN) and the primary visual cortex (V1) alongside a increase in individual whisker barrel size. Neocortical gene expression in the aging brain has not been previously identified; we document cortical expression of multiple regionalization genes. Expression patterns of Ephrin A5, COUP-TFI, and RZRβ and patterns of intraneocortical connectivity (INC) are altered in the neocortices of aging blind mice. Sensory inputs from different modalities during development likely play a major role in the development of cortical areal and thalamic nuclear boundaries. We suggest that early patterning by prenatal retinal activity combined with persistent gene expression within the thalamus and cortex is sufficient to establish and preserve a small but present LGN and V1 into late adulthood.

The classic cadherins in synaptic specificity

During brain development, billions of neurons organize into highly specific circuits. To form specific circuits, neurons must build the appropriate types of synapses with appropriate types of synaptic partners while avoiding incorrect partners in a dense cellular environment. Defining the cellular and molecular rules that govern specific circuit formation has significant scientific and clinical relevance because fine scale connectivity defects are thought to underlie many cognitive and psychiatric disorders. Organizing specific neural circuits is an enormously complicated developmental process that requires the concerted action of many molecules, neural activity, and temporal events. This review focuses on one class of molecules postulated to play an important role in target selection and specific synapse formation: the classic cadherins. Cadherins have a well-established role in epithelial cell adhesion, and although it has long been appreciated that most cadherins are expressed in the brain, their role in synaptic specificity is just beginning to be unraveled. Here, we review past and present studies implicating cadherins as active participants in the formation, function, and dysfunction of specific neural circuits and pose some of the major remaining questions.

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.

CDH2 and CDH11 act as regulators of stem cell fate decisions

Accumulating evidence suggests that the mechanical and biochemical signals originating from cell-cell adhesion are critical for stem cell lineage specification. In this review, we focus on the role of cadherin mediated signaling in development and stem cell differentiation, with emphasis on two well-known cadherins, cadherin-2 (CDH2) (N-cadherin) and cadherin-11 (CDH11) (OB-cadherin). We summarize the existing knowledge regarding the role of CDH2 and CDH11 during development and differentiation in vivo and in vitro. We also discuss engineering strategies to control stem cell fate decisions by fine-tuning the extent of cell-cell adhesion through surface chemistry and microtopology. These studies may be greatly facilitated by novel strategies that enable monitoring of stem cell specification in real time. We expect that better understanding of how intercellular adhesion signaling affects lineage specification may impact biomaterial and scaffold design to control stem cell fate decisions in three-dimensional context with potential implications for tissue engineering and regenerative medicine.

Cadherin-based transsynaptic networks in establishing and modifying neural connectivity

It is tacitly understood that cell adhesion molecules (CAMs) are critically important for the development of cells, circuits, and synapses in the brain. What is less clear is what CAMs continue to contribute to brain structure and function after the early period of development. Here, we focus on the cadherin family of CAMs to first briefly recap their multidimensional roles in neural development and then to highlight emerging data showing that with maturity, cadherins become largely dispensible for maintaining neuronal and synaptic structure, instead displaying new and narrower roles at mature synapses where they critically regulate dynamic aspects of synaptic signaling, structural plasticity, and cognitive function. At mature synapses, cadherins are an integral component of multiprotein networks, modifying synaptic signaling, morphology, and plasticity through collaborative interactions with other CAM family members as well as a variety of neurotransmitter receptors, scaffolding proteins, and other effector molecules. Such recognition of the ever-evolving functions of synaptic cadherins may yield insight into the pathophysiology of brain disorders in which cadherins have been implicated and that manifest at different times of life.

The role of T-cadherin in axonal pathway formation in neocortical circuits

Cortical efferent and afferent fibers are arranged in a stereotyped pattern in the intermediate zone (IZ). Here, we studied the mechanism of axonal pathway formation by identifying a molecule that is expressed in a subset of cortical axons in the rat. We found that T-cadherin (T-cad), a member of the cadherin family, is expressed in deep-layer cell axons projecting to subcortical structures, but not in upper layer callosal axons projecting to the contralateral cortex. Ectopic expression of T-cad in upper layer cells induced axons to project toward subcortical structures via the upper part of the IZ. Moreover, the axons of deep-layer cells in which T-cad expression was suppressed by RNAi projected towards the contralateral cortex via an aberrant route. These results suggest that T-cad is involved in axonal pathway formation in the developing cortex.

Central topography of cranial motor nuclei controlled by differential cadherin expression

Neuronal nuclei are prominent, evolutionarily conserved features of vertebrate central nervous system (CNS) organization [1]. Nuclei are clusters of soma of functionally related neurons and are located in highly stereotyped positions. Establishment of this CNS topography is critical to neural circuit assembly. However, little is known of either the cellular or molecular mechanisms that drive nucleus formation during development, a process termed nucleogenesis [2, 3, 4, 5]. Brainstem motor neurons, which contribute axons to distinct cranial nerves and whose functions are essential to vertebrate survival, are organized exclusively as nuclei. Cranial motor nuclei are composed of two main classes, termed branchiomotor/visceromotor and somatomotor [6]. Each of these classes innervates evolutionarily distinct structures, for example, the branchial arches and eyes, respectively. Additionally, each class is generated by distinct progenitor cell populations and is defined by differential transcription factor expression [7, 8]; for example, Hb9 distinguishes somatomotor from branchiomotor neurons. We characterized the time course of cranial motornucleogenesis, finding that despite differences in cellular origin, segregation of branchiomotor and somatomotor nuclei occurs actively, passing through a phase of each being intermingled. We also found that differential expression of cadherin cell adhesion family members uniquely defines each motor nucleus. We show that cadherin expression is critical to nucleogenesis as its perturbation degrades nucleus topography predictably.

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Cranial motor nucleogenesis occurs through an active process of segregation

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Differential cadherin expression defines cranial motor nuclei

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Cadherin expression drives specificity of cranial motor nucleus segregation

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Cadherin expression does not affect cranial motor neuron migration

Netrin-4 regulates thalamocortical axon branching in an activity-dependent fashion

Axon branching is remodeled by sensory-evoked and spontaneous neuronal activity. However, the underlying molecular mechanism is largely unknown. Here, we demonstrate that the netrin family member netrin-4 (NTN4) contributes to activity-dependent thalamocortical (TC) axon branching. In the postnatal developmental stages of rodents, ntn4 expression was abundant in and around the TC recipient layers of sensory cortices. Neuronal activity dramatically altered the ntn4 expression level in the cortex in vitro and in vivo. TC axon branching was promoted by exogenous NTN4 and suppressed by depletion of the endogenous protein. Moreover, unc-5 homolog B (Unc5B), which strongly bound to NTN4, was expressed in the sensory thalamus, and knockdown of Unc5B in thalamic cells markedly reduced TC axon branching. These results suggest that NTN4 acts as a positive regulator for TC axon branching through activity-dependent expression.

Cadherin-8 expression, synaptic localization, and molecular control of neuronal form in prefrontal corticostriatal circuits

Neocortical interactions with the dorsal striatum support many motor and executive functions, and such underlying functional networks are particularly vulnerable to a variety of developmental, neurological, and psychiatric brain disorders, including autism spectrum disorders, Parkinson's disease, and Huntington's disease. Relatively little is known about the development of functional corticostriatal interactions, and in particular, virtually nothing is known of the molecular mechanisms that control generation of prefrontal cortex-striatal circuits. Here, we used regional and cellular in situ hybridization techniques coupled with neuronal tract tracing to show that Cadherin-8 (Cdh8), a homophilic adhesion protein encoded by a gene associated with autism spectrum disorders and learning disability susceptibility, is enriched within striatal projection neurons in the medial prefrontal cortex and in striatal medium spiny neurons forming the direct or indirect pathways. Developmental analysis of quantitative real-time polymerase chain reaction and western blot data show that Cdh8 expression peaks in the prefrontal cortex and striatum at P10, when cortical projections start to form synapses in the striatum. High-resolution immunoelectron microscopy shows that Cdh8 is concentrated at excitatory synapses in the dorsal striatum, and Cdh8 knockdown in cortical neurons impairs dendritic arborization and dendrite self-avoidance. Taken together, our findings indicate that Cdh8 delineates developing corticostriatal circuits where it is a strong candidate for regulating the generation of normal cortical projections, neuronal morphology, and corticostriatal synapses.

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.

Cadherins: potential regulators in the faculty of language

The cadherin superfamily is a large (now more than 100 proteins) protein family originally identified as cell adhesion molecules. Each cadherin shows distinct expression patterns in the nervous system, and their expressions are both spatially and temporally regulated and diverse among different species. Mounting evidence has suggested that cadherins play multiple roles in neural development and functions. Recently, using songbirds and mice, the potential role of cadherins in vocal behavior has been demonstrated. Here, we will briefly introduce general function of cadherins, and analyze the potential involvement of cadherins in vocal behaviors and their evolution.

Spinal motor neuron migration and the significance of topographic organization in the nervous system

The nervous system displays a high degree of topographic organisation such that neuronal soma position is closely correlated to axonal trajectory. One example of such order is the myotopic organisation of the motor system where spinal motor neuron position parallels that of target muscles. This chapter will discuss the molecular mechanisms underlying motor neuron soma positioning, which include transcriptional control of Reelin signaling and cadherin expression. As the same transcription factors have been shown to control motor axon innervation of target muscles, a simple mechanism of topographic organisation specification is becoming evident raising the question of how coordinating soma position with axon trajectory might be important for nervous system wiring and its function.

Cortical plasticity within and across lifetimes: how can development inform us about phenotypic transformations?

The neocortex is the part of the mammalian brain that is involved in perception, cognition, and volitional motor control. It is a highly dynamic structure that is dramatically altered within the lifetime of an animal and in different lineages throughout the course of evolution. These alterations account for the remarkable variations in behavior that species exhibit. Of particular interest is how these cortical phenotypes change within the lifetime of the individual and eventually evolve in species over time. Because we cannot study the evolution of the neocortex directly we use comparative analysis to appreciate the types of changes that have been made to the neocortex and the similarities that exist across taxa. Developmental studies inform us about how these phenotypic transitions may arise by alterations in developmental cascades or changes in the physical environment in which the brain develops. Both genes and the sensory environment contribute to aspects of the phenotype and similar features, such as the size of a cortical field, can be altered in a variety of ways. Although both genes and the laws of physics place constraints on the evolution of the neocortex, mammals have evolved a number of mechanisms that allow them to loosen these constraints and often alter the course of their own evolution.

Comparative analysis of protocadherin-11 X-linked expression among postnatal rodents, non-human primates, and songbirds suggests its possible involvement in brain evolution

Background

Protocadherin-11 is a cell adhesion molecule of the cadherin superfamily. Since, only in humans, its paralog is found on the Y chromosome, it is expected that protocadherin-11X/Y plays some role in human brain evolution or sex differences. Recently, a genetic mutation of protocadherin-11X/Y was reported to be associated with a language development disorder. Here, we compared the expression of protocadherin-11 X-linked in developing postnatal brains of mouse (rodent) and common marmoset (non-human primate) to explore its possible involvement in mammalian brain evolution. We also investigated its expression in the Bengalese finch (songbird) to explore a possible function in animal vocalization and human language faculties.

Methodology/Principal Findings

Protocadherin-11 X-linked was strongly expressed in the cerebral cortex, hippocampus, amygdala and brainstem. Comparative analysis between mice and marmosets revealed that in certain areas of marmoset brain, the expression was clearly enriched. In Bengalese finches, protocadherin-11 X-linked was expressed not only in nuclei of regions of the vocal production pathway and the tracheosyringeal hypoglossal nucleus, but also in areas homologous to the mammalian amygdala and hippocampus. In both marmosets and Bengalese finches, its expression in pallial vocal control areas was developmentally regulated, and no clear expression was seen in the dorsal striatum, indicating a similarity between songbirds and non-human primates.

Conclusions/Significance

Our results suggest that the enriched expression of protocadherin-11 X-linked is involved in primate brain evolution and that some similarity exists between songbirds and primates regarding the neural basis for vocalization.

Architecture and development of olivocerebellar circuit topography

The cerebellum has a simple tri-laminar structure that is comprised of relatively few cell types. Yet, its internal micro-circuitry is anatomically, biochemically, and functionally complex. The most striking feature of cerebellar circuit complexity is its compartmentalized topography. Each cell type within the cerebellar cortex is organized into an exquisite map; molecular expression patterns, dendrite projections, and axon terminal fields divide the medial-lateral axis of the cerebellum into topographic sagittal zones. Here, we discuss the mechanisms that establish zones and highlight how gene expression and neural activity contribute to cerebellar pattern formation. We focus on the olivocerebellar system because its developmental mechanisms are becoming clear, its topographic termination patterns are very precise, and its contribution to zonal function is debated. This review deconstructs the architecture and development of the olivocerebellar pathway to provide an update on how brain circuit maps form and function.

Regulation of cadherin expression in nervous system development

This review addresses our current understanding of the regulatory mechanisms for classical cadherin expression during development of the vertebrate nervous system. The complexity of the spatial and temporal expression patterns is linked to morphogenic and functional roles in the developing nervous system. While the regulatory networks controlling cadherin expression are not well understood, it is likely that the multiple signaling pathways active in the development of particular domains also regulate the specific cadherins expressed at that time and location. With the growing understanding of the broader roles of cadherins in cell-cell adhesion and non-adhesion processes, it is important to understand both the upstream regulation of cadherin expression and the downstream effects of specific cadherins within their cellular context.