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.     

Molecular biology of cadherins in the nervous system

Cadherins are cell-cell adhesion molecules belonging to the Ca²⁺-dependent cadherin superfamily. In the last few years the number of cadherins identified in the nervous system has increased considerably. Cadherins are integral membrane glycoproteins. They are structurally closely related and interspecies homologies are high. The function is mediated through a homophilic binding mechanism, and intracellular proteins, directly or indirectly connected to the cadherins and the cytoskeleton, are necessary for cadherin activity. Cadherins have been implicated in segregation and aggregation of tissues at early developmental stages and in growth and guidance of axons during nervous system development. These functions are modified by changes in type(s) and amount of cadherins expressed at different developmental stages. The regulatory elements guiding cadherin expression are currently being elucidated.     

Cadherins in the Developing Central Nervous System: An Adhesive Code for Segmental and Functional Subdivisions

In this review, we describe general features of the expression of cadherins in the developing central nervous system (CNS) of vertebrates. In the early neuroepithelium, the expression of several cadherins is restricted to specific regions corresponding to segmental domains. Segmental boundaries often coincide with changes in cadherin expression, subdividing the primordial CNS into different adhesive domains. In the different neuromeric domains, early neurons are generated which differentially express cadherins. In the mantle layer, these early neurons seem to sort out according to which cadherin they express, and they aggregate into various gray matter regions (brain nuclei and cortical lamina and regions). The gray matter structures expressing a given cadherin become connected to one another to form parts of particular functional systems or neuronal circuits. Together, these findings show that cadherins provide a molecular system reflecting both early embryonic and mature nervous system architecture. The possible roles of cadherins in the formation and maintenance of segmental and functional nervous system structures are discussed.     

Cadherins in neuronal morphogenesis and function

Classic cadherins represent a family of calcium-dependent homophilic cell–cell adhesion molecules. They confer strong adhesiveness to animal cells when they are anchored to the actin cytoskeleton via their cytoplasmic binding partners, catenins. The cadherin/catenin adhesion system plays key roles in the morphogenesis and function of the vertebrate and invertebrate nervous systems. In early vertebrate development, cadherins are involved in multiple events of brain morphogenesis including the formation and maintenance of the neuroepithelium, neurite extension and migration of neuronal cells. In the invertebrate nervous system, classic cadherin-mediated cell–cell interaction plays important roles in wiring among neurons. For synaptogenesis, the cadherin/catenin system not only stabilizes cell–cell contacts at excitatory synapses but also assembles synaptic molecules at synaptic sites. Furthermore, this system is involved in synaptic plasticity. Recent studies on the role of individual cadherin subtypes at synapses indicate that individual cadherin subtypes play their own unique role to regulate synaptic activities.     

To adhere or not to adhere: The role of Cadherins in neural crest development

The modulation of cell adhesion is fundamental to the morphogenesis that accompanies proper embryonic development. Cadherins are a large family of calcium-dependent cell adhesion molecules whose spatial and temporal expression is critical to the formation of the neural crest, a unique, multipotent cell type that contributes to the patterning of the vertebrate body plan. Neural crest cells arise from the embryonic ectoderm through inductive interactions and reside in the dorsal aspect of the neural tube. These cells under go an epithelial-to-mesenchymal transition and migrate to precise destinations in the embryo, where they go on to differentiate into such diverse structures as melanocytes, elements of the peripheral nervous system and the craniofacial skeleton. Distinct cadherins are expressed during the induction, migration and differentiation of the neural crest. With the advent of genomic sequencing, assembly and annotation for various model organisms, it has become possible to elucidate the molecular mechanisms underlying cadherin expression, and how these cadherins function, during neural crest development. This review explores the known roles of cadherins and details, where relevant, how different cadherins are regulated during the formation of the neural crest.Key words: cadherins, neural crest, EMT, induction, migration, differentiation     

To adhere or not to adhere

The modulation of cell adhesion is fundamental to the morphogenesis that accompanies proper embryonic development. Cadherins are a large family of calcium-dependent cell adhesion molecules whose spatial and temporal expression is critical to the formation of the neural crest, a unique, multipotent cell type that contributes to the patterning of the vertebrate body plan. Neural crest cells arise from the embryonic ectoderm through inductive interactions and reside in the dorsal aspect of the neural tube. These cells under go an epithelial-to-mesenchymal transition and migrate to precise destinations in the embryo, where they go on to differentiate into such diverse structures as melanocytes, elements of the peripheral nervous system, and the craniofacial skeleton. Distinct cadherins are expressed during the induction, migration and differentiation of the neural crest. With the advent of genomic sequencing, assembly and annotation for various model organisms, it has become possible to elucidate the molecular mechanisms underlying cadherin expression, and how these cadherins function, during neural crest development. This review explores the known roles of cadherins and details, where relevant, how different cadherins are regulated during the formation of the neural crest.     

Should I stay or should I go? Cadherin function and regulation in the neural crest

Our increasing comprehension of neural crest cell development has reciprocally advanced our understanding of cadherin expression, regulation, and function. As a transient population of multipotent stem cells that significantly contribute to the vertebrate body plan, neural crest cells undergo a variety of transformative processes and exhibit many cellular behaviors, including epithelial‐to‐mesenchymal transition (EMT), motility, collective cell migration, and differentiation. Multiple studies have elucidated regulatory and mechanistic details of specific cadherins during neural crest cell development in a highly contextual manner. Collectively, these results reveal that gradual changes within neural crest cells are accompanied by often times subtle, yet important, alterations in cadherin expression and function. The primary focus of this review is to coalesce recent data on cadherins in neural crest cells, from their specification to their emergence as motile cells soon after EMT, and to highlight the complexities of cadherin expression beyond our current perceptions, including the hypothesis that the neural crest EMT is a transition involving a predominantly singular cadherin switch. Further advancements in genetic approaches and molecular techniques will provide greater opportunities to integrate data from various model systems in order to distinguish unique or overlapping functions of cadherins expressed at any point throughout the ontogeny of the neural crest.     

Classic cadherins regulate tangential migration of precerebellar neurons in the caudal hindbrain

Classic cadherins are calcium dependent homophilic cell adhesion molecules that play a key role in developmental processes such as morphogenesis, compartmentalization and maintenance of a tissue. They also play important roles in development and function of the nervous system. Although classic cadherins have been shown to be involved in the migration of non-neuronal cells, little is known about their role in neuronal migration. Here, we show that classic cadherins are essential for the migration of precerebellar neurons. In situ hybridization analysis shows that at least four classic cadherins, cadherin 6 (Cad6), cadherin 8 (Cad8), cadherin11 (Cad11) and N-cadherin (Ncad), are expressed in the migratory streams of lateral reticular nucleus and external cuneate nucleus (LRN/ECN) neurons. Functional analysis performed by electroporation of cadherin constructs into the hindbrain indicates requirement for cadherins in the migration of LRN/ECN neurons both in vitro and in vivo. While overexpression of full-length classic cadherins, NCAD and CAD11, has no effect on LRN/ECN neuron migration, overexpression of two dominant negative (DN) constructs, membrane-bound form and cytoplasmic form, slows it down. Introduction of a DN construct does not alter some characteristics of LRN/ECN cells as indicated by a molecular marker, TAG1, and their responsiveness to chemotropic activity of the floor plate (FP). These results suggest that classic cadherins contribute to contact-dependent mechanisms of precerebellar neuron migration probably via their adhesive property.     

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.     

Seven-Pass Transmembrane Cadherins: Roles and Emerging Mechanisms in Axonal and Dendritic Patterning

The Flamingo/Celsr seven-transmembrane cadherins represent a conserved subgroup of the cadherin superfamily involved in multiple aspects of development. In the developing nervous system, Fmi/Celsr control axonal blueprint and dendritic morphogenesis from invertebrates to mammals. As expected from their molecular structure, seven-transmembrane cadherins can induce cell–cell homophilic interactions but also intracellular signaling. Fmi/Celsr is known to regulate planar cell polarity (PCP) through interactions with PCP proteins. In the nervous system, Fmi/Celsr can function in collaboration with or independently of other PCP genes. Here, we focus on recent studies which show that seven-transmembrane cadherins use distinct molecular mechanisms to achieve diverse functions in the development of the nervous system.     

Cadherins and catenins in dendrite and synapse morphogenesis

Neurons are highly polarized specialized cells. Neuronal integrity and functional roles are critically dependent on dendritic architecture and synaptic structure, function and plasticity. The cadherins are glycosylated transmembrane proteins that form cell adhesion complexes in various tissues. They are associated with a group of cytosolic proteins, the catenins. While the functional roles of the complex have been extensively investigates in non-neuronal cells, it is becoming increasingly clear that components of the complex have critical roles in regulating dendritic and synaptic architecture, function and plasticity in neurons. Consistent with these functional roles, aberrations in components of the complex have been implicated in a variety of neurodevelopmental disorders. In this review, we discuss the roles of the classical cadherins and catenins in various aspects of dendrite and synapse architecture and function and their relevance to human neurological disorders. Cadherins are glycosylated transmembrane proteins that were initially identified as Ca²⁺-dependent cell adhesion molecules. They are present on plasma membrane of a variety of cell types from primitive metazoans to humans. In the past several years, it has become clear that in addition to providing mechanical adhesion between cells, cadherins play integral roles in tissue morphogenesis and homeostasis. The cadherin family is composed of more than 100 members and classified into several subfamilies, including classical cadherins and protocadherins. Several of these cadherin family members have been implicated in various aspects of neuronal development and function.^1-3 The classical cadherins are associated with a group of cytosolic proteins, collectively called the catenins. While the functional roles of the cadherin-catenin cell adhesion complex have been extensively investigated in epithelial cells, it is now clear that components of the complex are well expressed in central neurons at different stages during development.^4,5 Recent exciting studies have shed some light on the functional roles of cadherins and catenins in central neurons. In this review, we will provide a brief overview of the cadherin superfamily, describe cadherin family members expressed in central neurons, cadherin-catenin complexes in central neurons and then focus on role of the cadherin-catenin complex in dendrite morphogenesis and synapse morphogenesis, function and plasticity. The final section is dedicated to discussion of the emerging list of neural disorders linked to cadherins and catenins. While the roles of cadherins and catenins have been examined in several different types of neurons, the focus of this review is their role in mammalian central neurons, particularly those of the cortex and hippocampus. Accompanying this review is a series of excellent reviews targeting the roles of cadherins and protocadherins in other aspects of neural development.     

Structure of the cadherin-related neuronal receptor/protocadherin-alpha first extracellular cadherin domain reveals diversity across cadherin families

The recent explosion in genome sequencing has revealed the great diversity of the cadherin superfamily. Within the superfamily, protocadherins, which are expressed mainly in the nervous system, constitute the largest subgroup. Nevertheless, the structures of only the classical cadherins are known. Thus, to broaden our understanding of the adhesion repertoire of the cadherin superfamily, we determined the structure of the N-terminal first extracellular cadherin domain of the cadherin-related neuronal receptor/protocadherin-alpha4. The hydrophobic pocket essential for homophilic adhesiveness in the classical cadherins was not found, and the functional significance of this structural domain was supported by exchanging the first extracellular cadherin domains of protocadherin and classical cadherin. Moreover, potentially crucial variations were observed mainly in the loop regions. These included the protocadherin-specific disulfide-bonded Cys-X(5)-Cys motif, which showed Ca(2+)-induced chemical shifts, and the RGD motif, which has been suggested to be involved in heterophilic cell adhesion via the active form of beta1 integrin. Our findings reveal that the adhesion repertoire of the cadherin superfamily is far more divergent than would be predicted by studying the classical cadherins alone.     

Complementary and dynamic type II cadherin expression associated with development of the primate visual system

The middle temporal visual area (MT, also known as V5) is a visual association area that is particularly evolved in the primate brain. The MT receives input from the primary visual area (V1), constitutes part of the dorsal visual pathway, and plays an essential role in processing motion. Connections between the MT and V1 in the primate brain are formed after birth, and are related to the maturation of visual system. However, it remains to be determined what molecular mechanisms control the formation and maturation of the visual system. Cadherins are transmembrane proteins, originally isolated as cell adhesion molecules, which have multiple roles in synapse formation and function. To investigate potential involvement of cadherins in development of the primate visual system, we examined type II cadherin expression (cadherin‐6, ‐8, ‐12) in cortical and thalamic visual areas of pre‐ and postnatal brains of the common marmoset (Callithrix jacchus). In the prenatal brain, cadherin‐6 was dominantly expressed in the pulvino‐MT pathway whereas cadherin‐8 was dominant in the lateral geniculate nucleus (LGN)‐V1 pathway. During postnatal development, there was a downregulation of cadherin‐6 and upregulation of cadherin‐8 expression in the MT. The timing of this cadherin exchange preceded the development of V1‐MT connections. Our results suggest the possibility that changes in cadherin expression are involved in the development of the primate visual system, and that a switch in cadherin expression may be a general mechanism to control neural plasticity of highly cognitive abilities.     

Expression of cadherin10, a type II classic cadherin gene, in the nervous system of the embryonic zebrafish

Cadherins are cell surface adhesion molecules that play important roles in development of tissues and organs. In this study, we analyzed expression pattern of cadherin10, a member of the type II classic cadherin subfamily, in the embryonic zebrafish using in situ hybridization methods. cadherin10 message (cdh10) is first and transiently expressed by the notochord. In the developing nervous system, cdh10 was first detected in a subset of the cranial ganglia, then in restricted brain regions and neural retina. As development proceeds, cdh10 expression domain and/or expression levels increased in the embryonic nervous system. Our results show that cdh10 expression in the zebrafish developing nervous system is both spatially and temporally regulated.     

Diversity of the cadherin family: evidence for eight new cadherins in nervous tissue.

To examine the diversity of the cadherin family, we isolated cDNAs from brain and retina cDNA preparations with the aid of polymerase chain reaction. The products obtained included cDNAs for two of three known cadherins as well as eight distinct cDNAs, of which deduced amino acid sequences show significant similarity with the known cadherin sequences. Larger cDNA clones were isolated from human cDNA libraries for six of the eight new molecules. The deduced amino acid sequences show that the overall structure of these molecules is very similar to that of the known cadherins, indicating that these molecules are new members of the cadherin family. We have tentatively designated these cadherins as cadherin-4 through -11. The new molecules, with the exception of cadherin-4, exhibit features that distinguish them as a group from previously cloned cadherins; they may belong to a new subfamily of cadherins. Northern blot analysis showed that most of these cadherins are expressed mainly in brain, although some are expressed in other tissues as well. These findings show that the cadherin family of adhesion molecules is much larger than previously thought, and suggest that the new cadherins may play an important role in cell-cell interactions within the central nervous system.     

Co-operative roles for E-cadherin and N-cadherin during lens vesicle separation and lens epithelial cell survival

The classical cadherins are known to have both adhesive and signaling functions. It has also been proposed that localized regulation of cadherin activity may be important in cell assortment during development. In the context of eye development, it has been suggested that cadherins are important for separation of the invaginated lens vesicle from the surface ectoderm. To test this hypothesis, we conditionally deleted N-cadherin or E-cadherin from the presumptive lens ectoderm of the mouse. Conditional deletion of either cadherin alone did not produce a lens vesicle separation defect. However, these conditional mutants did exhibit common structural deficits, including microphthalmia, severe iris hyperplasia, persistent vacuolization within the fibre cell region, and eventual lens epithelial cell deterioration. To assess the co-operative roles of E-cadherin and N-cadherin within the developing lens, double conditional knockout embryos were generated. These mice displayed distinct defects in lens vesicle separation and persistent expression of another classical cadherin, P-cadherin, within the cells of the persistent lens stalk. Double mutant lenses also exhibited severe defects in lens epithelial cell adhesion and survival. Finally, the severity of the lens phenotype was shown to be sensitive to the number of wild-type E- and N-cadherin alleles. These data suggest that the co-operative expression of both E- and N-cadherin during lens development is essential for normal cell sorting and subsequent lens vesicle separation.     

Distinct roles of cadherin-6 and E-cadherin in tubulogenesis and lumen formation

Classic cadherins are important regulators of tissue morphogenesis. The predominant cadherin in epithelial cells, E-cadherin, has been extensively studied because of its critical role in normal epithelial development and carcinogenesis. Epithelial cells may also coexpress other cadherins, but their roles are less clear. The Madin Darby canine kidney (MDCK) cell line has been a popular mammalian model to investigate the role of E-cadherin in epithelial polarization and tubulogenesis. However, MDCK cells also express relatively high levels of cadherin-6, and it is unclear whether the functions of this cadherin are redundant to those of E-cadherin. We investigate the specific roles of both cadherins using a knockdown approach. Although we find that both cadherins are able to form adherens junctions at the basolateral surface, we show that they have specific and mutually exclusive roles in epithelial morphogenesis. Specifically, we find that cadherin-6 functions as an inhibitor of tubulogenesis, whereas E-cadherin is required for lumen formation. Ablation of cadherin-6 leads to the spontaneous formation of tubules, which depends on increased phosphoinositide 3-kinase (PI3K) activity. In contrast, loss of E-cadherin inhibits lumen formation by a mechanism independent of PI3K.     

Differential expression of E-cadherin,N-cadherin and beta-catenin in proximal and distal segments of the rat nephron.

Background  

The classical cadherins such as E- and N-cadherin are Ca²⁺-dependent cell adhesion molecules that play important roles in the development and maintenance of renal epithelial polarity. Recent studies have shown that a variety of cadherins are present in the kidney and are differentially expressed in various segments of the nephron. However, the interpretation of these findings has been complicated by the fact that the various studies focused on different panels of cadherins and utilized different species. Moreover, since only a few of the previous studies focused on the rat, information regarding the expression and localization of renal cadherins in this important species is lacking. In the present study, we have employed dual immunofluorescent labeling procedures that utilized specific antibodies against either E- or N-cadherin, along with antibodies that target markers for specific nephron segments, to characterize the patterns of cadherin expression in frozen sections of adult rat kidney.     

Non-clustered protocadherin

The cadherin family is classified into classical cadherins, desmosomal cadherins and protocadherins (PCDHs). Genomic structures distinguish between PCDHs and other cadherins, and between clustered and non-clustered PCDHs. The phylogenetic analysis with full sequences of non-clustered PCDHs enabled them to be further classified into three subgroups: δ1 (PCDH1, PCDH7, PCDH9, PCDH11 and PCDH20), δ2 (PCDH8, PCDH10, PCDH12, PCDH17, PCDH18 and PCDH19) and ε (PCDH15, PCDH16, PCDH21 and MUCDHL). ε-PCDH members except PCDH21 have either higher or lower numbers of cadherin repeats than those of other PCDHs. Non-clustered PCDHs are expressed predominantly in the nervous system and have spatiotemporally diverse expression patterns. Especially, the region-specific expressions of non-clustered PCDHs have been observed in cortical area of early postnatal stage and in caudate putaman and/or hippocampal formation of mature brains, suggesting that non-clustered PCDHs play roles in the circuit formation and maintenance. The non-clustered PCDHs appear to have homophilic/heterophilic cell-cell adhesion properties, and each member has diverse cell signaling partnership distinct from those of other members (PCDH7/TAF1; PCDH8/TAO2δ; PCDH10/Nap1; PCDH11/δ-catenin; PCDH18/mDab1). Furthermore, each PCDH has several isoforms with differential cytoplasmic sequences, suggesting that one PCDH isoform could activate intracellular signaling differential from other isoforms. These facts suggest that non-clustered PCDHs play roles as a mediator of a regulator of other molecules as well as cell-cell adhesion. Furthermore, some non-clustered PCDHs have been considered to be involved in neuronal diseases such as autism-spectrum disorders, schizophrenia and female-limited epilepsy and cognitive impairment, suggesting that they play multiple, tightly regulated roles in normal brain function. In addition, some non-clustered PCDHs have been suggested as candidate tumor suppressor genes in several tissues. Although molecular adhesive and regulatory properties of some PCDHs began to be unveiled, the endeavor to understand the molecular mechanism of non-clustered PCDH is still in its infancy and requires future study.Key words: non-clustered protocadherins, gene structure, cell adhesion, intracellular signaling, neural disease, cancer