Plasticity of Cadherin–Catenin Expression in the Melanocyte Lineage

Cadherins are calcium‐dependent cell adhesion receptors with strong morphoregulatory functions. To mediate functional adhesion, cadherins must interact with actin cytoskeleton. Catenins are cytoplasmic proteins that mediate the interactions between cadherins and the cytoskeleton. In addition to their role in cell–cell adhesion, catenins also participate in signaling pathways that regulate cell growth and differentiation. Cadherins and catenins appear to be involved in melanocyte development and transformation. Here, we investigated the function of cadherin–catenin complexes in the normal development and transformation of melanocytes by studying the patterns of expression of the cell–cell adhesion molecules, E‐, N‐ and P‐cadherin, and the expression of their cytoplasmic partners, α‐, β‐ and Γ‐catenin, during murine development. Similar analyses were performed in vitro using murine melanoblast, melanocyte, and melanoma cell lines in the presence and absence of keratinocytes, the cells with which melanocytes interact in vivo. Overall, the results suggest that the expression of cadherins and catenins is very plastic and depends on their environment as well as the transformation status of the cells. This plasticity is important in fundamental cellular mechanisms associated with normal and pathological ontogenesis, as well as with tumorigenesis.     

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.     

Making and breaking contacts: the cellular biology of cadherin regulation

Cadherin-mediated cell-cell interactions are dynamic processes, and cadherin function is tightly regulated in response to cellular context and signaling. Ultimately, cadherin regulation is likely to reflect the interplay between a range of fundamental cellular processes, including surface organization of receptors, cytoskeletal organization and cell trafficking, that are coordinated by signaling events. In this review we focus on recent advances in understanding how interplay with membrane trafficking and other cell-cell junctions can control cadherin function. The endocytosis of cadherins, and their post-internalization fate, influences surface expression and metabolic stability of these adhesion receptors. Similarly, at the surface, components of tight junctions provide a mode of cross-talk that regulates assembly of adherens junctions.     

Cadherins in development and cancer

Proper embryonic development is guaranteed under conditions of regulated cell-cell and cell-matrix adhesion. The cells of an embryo have to be able to distinguish their neighbours as being alike or different. Cadherins, single-pass transmembrane, Ca(2+)-dependent adhesion molecules that mainly interact in a homophilic manner, are major contributors to cell-cell adhesion. Cadherins play pivotal roles in important morphogenetic and differentiation processes during development, and in maintaining tissue integrity and homeostasis. Changes in cadherin expression throughout development enable differentiation and the formation of various organs. In addition to these functions, cadherins have strong implications in tumourigenesis, since frequently tumour cells show deregulated cadherin expression and inappropriate switching among family members. In this review, I focus on E- and N-cadherin, giving an overview of their structure, cellular function, importance during development, role in cancer, and of the complexity of Ecadherin gene regulation.     

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.     

Computer simulations of cell sorting due to differential adhesion

The actions of cell adhesion molecules, in particular, cadherins during embryonic development and morphogenesis more generally, regulate many aspects of cellular interactions, regulation and signaling. Often, a gradient of cadherin expression levels drives collective and relative cell motions generating macroscopic cell sorting. Computer simulations of cell sorting have focused on the interactions of cells with only a few discrete adhesion levels between cells, ignoring biologically observed continuous variations in expression levels and possible nonlinearities in molecular binding. In this paper, we present three models relating the surface density of cadherins to the net intercellular adhesion and interfacial tension for both discrete and continuous levels of cadherin expression. We then use then the Glazier-Graner-Hogeweg (GGH) model to investigate how variations in the distribution of the number of cadherins per cell and in the choice of binding model affect cell sorting. We find that an aggregate with a continuous variation in the level of a single type of cadherin molecule sorts more slowly than one with two levels. The rate of sorting increases strongly with the interfacial tension, which depends both on the maximum difference in number of cadherins per cell and on the binding model. Our approach helps connect signaling at the molecular level to tissue-level morphogenesis.     

Cadherins in the mammary gland and the melanocyte lineage

Cadherins are transmembrane glycoproteins involved in cell-cell adhesion, signalling, proliferation and differentiation. In this review, we have focused upon in vivo cadherin expression and function in two different biological systems, the mammary gland epithelium and the melanocyte lineage. Development of the mammary gland represents a paradigm of in situ epithelial differentiation and the melanocyte lineage of a model of non-epithelial (or mesenchymal) cell differentiation where cells migrate extensively from their site of origin towards the skin compartment. In the mammary epithelium, the predominantly expressed cadherin is E-cadherin, a cell surface molecule that directs morphogenesis and maintenance of the epithelial structure. In the melanocyte lineage, the expression of a number of cadherins is strictly spatiotemporally regulated during development and adult life. The specific functions mediated by this very dynamic cadherin expression are not yet known and their characterisation represents a challenge for the future.     

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.     

Disruption of epithelial cell-cell adhesion by exogenous expression of a mutated nonfunctional N-cadherin.

Cadherins, a family of transmembrane cell-cell adhesion receptors, require interactions with the cytoskeleton for normal function. To assess the mechanisms of these interactions, we studied the effect of exogenous expression of a mutant N-cadherin, cN390 delta; on epithelial cell-cell adhesion. The intracellular domain of cN390 delta was intact but its extracellular domain was largely deleted so that this molecule was not functional for cell adhesion. cDNA of cN390 delta was attached to the metallothionein promoter, and introduced into the keratinocyte line PAM212 expressing endogenous E- and P-cadherin. When the expression of cN390 delta was induced by Zn2+, cadherin-dependent adhesion of the transfected cells was inhibited, resulting in the dispersion of cell colonies, although their contacts were maintained under high cell density conditions. In these cultures, cN390 delta was expressed not only on the free surfaces of the cells but also at cell-cell junctions. The endogenous cadherins were concentrated at cell-cell junctions under normal conditions. As a result of cN390 delta expression, however, the endogenous cadherins localizing at the cell-cell junctions were largely diminished, suggesting that these molecules were replaced by the mutant molecules at these sites. As a control, we transfected the same cell line with cDNA of a truncated form of N-cadherin cadherin whose intracellular C terminus had been deleted leaving the extracellular domain intact. This molecule had no effect on cell-cell adhesion, nor did it localize to cell-cell contact sites. We also found that the association of the endogenous cadherins with alpha- and beta-catenins and plakoglobin was not affected by the expression of cN390 delta, which also formed a complex with these molecules, suggesting that no competition occurred between the endogenous and exogenous cadherins for these cytoplasmic proteins. These and other additional results suggest that the nonfunctional cadherins whose intracellular domain is intact occupy the sites where the endogenous cadherins should localize, through interactions with the cytoskeleton, and inhibit the cadherin adhesion system.     

Transmembrane control of cadherin-mediated cell-cell adhesion

The cadherin family of cell-cell adhesion molecules plays a central role in organization of cells into multicellular structures. An important feature of the action of cadherins is that they form a complex with cytoskeletal proteins, and the formation of this complex is crucial for their adhesive function. Cadherin-mediated cell adhesion is thus controlled through the interaction with cytoplasmic proteins, and, for such control, phosphorylation of these proteins and also cadherins themselves might be involved. This regulatory mechanism of cell adhesion is perhaps fundamental to a variety of morphogenetic processes.     

Cadherins in vascular smooth muscle cell (patho)biology: Quid nos scimus?

Vascular smooth muscle cells (SMCs) phenotypes span a reversible continuum from quiescent/contractile (differentiated) to proliferative/synthetic (dedifferentiated) enabling them to perform a diversity of functions that are context-dependent and important for vascular tone-diameter homeostasis, vasculogenesis, angiogenesis or vessel reparation after injury. Dysregulated phenotype modulation and failure to maintain/regain the mature differentiated and contractile phenotypic state is pivotal in the development of vascular diseases such as atherosclerosis and restenosis after angioplasty and coronary bypass grafting. Many functions of SMCs such as adhesion, migration, proliferation, contraction, differentiation and apoptosis are regulated by a broad spectrum of cell-cell and cell-matrix adhesion molecules. Cadherins represent a superfamily of cell surface homophilic adhesion molecules with fundamental roles in morphogenetic and differentiation processes during development and in the maintenance of tissue integrity and homeostasis in adults. The cadherins have major inputs on signalling pathways and cytoskeletal assemblies that participate in regulating processes such as cell polarity, migration, proliferation, survival, phenotype and differentiation. Abnormalities in these processes have long been recognized to underlie pathological SMC-driven reparation, but knowledge on the involvement of cadherins is remarkably limited. This article presents a comprehensive review of cadherin family members currently identified on vascular SMCs in relation to their functions, molecular mechanisms of action and relevance for vascular pathology.     

Cadherins and synaptic plasticity

Given their trans-synaptic localization, their persistent expression at mature synapses and their distinct biochemical and adhesive properties, cadherins are uniquely poised at the synapse to mediate synaptic plasticity, the ability to change synaptic function thought to underlie learning and memory. For example recent work suggests that cadherins may recruit and stabilize AMPA receptors at the synapse via direct interactions or through complex formation, revealing cross talk between postsynaptic signaling and adhesion. Moreover, the use of small interfering RNA knockdown of cadherin, the availability of N-cadherin-deficient embryonic stem cells and the acute disruption of cadherin function with peptide application in vivo have allowed for more precise dissection of the molecular mechanisms by which cadherins function in both structural and functional plasticity.     

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.     

Endothelial cadherins in cancer

Cadherins are cell adhesion receptors that play important roles in embryogenesis and tissue homoeostasis. Endothelial cells express various members of the cadherin superfamily, in particular vascular endothelial (VE-) cadherin, which is the main adhesion receptor of endothelial adherens junctions and neural (N-) cadherin, which is normally localized outside the junctions and may mediate adhesion between endothelial cells and non-endothelial cells. Dysregulation of cadherin expression has been implicated in tumor progression, in particular the loss of epithelial (E-) cadherin expression or function and the gain of N-cadherin. Moreover, more recently, aberrant expression of VE-cadherin was observed in certain cancer types. In breast carcinoma, VE-cadherin was shown to promote tumor cell proliferation and invasion through enhancing TGF-β signaling. Thus, in breast cancer, the cadherin switch involves another player, vascular endothelial cadherin, which is part of an intricate interplay of classical cadherins in breast cancer progression.     

Tyrosine phosphorylation and cadherin/catenin function

Cadherin-mediated cell-cell adhesion is perturbed in protein tyrosine kinase (PTK)-transformed cells. While cadherins themselves appear to be poor PTK substrates, their cytoplasmic binding partners, the Arm catenins, are excellent PTK substrates and therefore good candidates for mediating PTK-induced changes in cadherin behavior. These proteins, p120^ctn, β-catenin and plakoglobin, bind to the cytoplasmic region of classical cadherins and function to modulate adhesion and/or bridge cadherins to the actin cytoskeleton. In addition, as demonstrated recently for β-catenin, these proteins also have crucial signaling roles that may or may not be related to their effects on cell-cell adhesion. Tyrosine phosphorylation of cadherin complexes is well documented and widely believed to modulate cell adhesiveness. The data to date, however, is largely correlative and the mechanism of action remains unresolved. In this review, we discuss the current literature and suggest models whereby tyrosine phosphorylation of Arm catenins contribute to regulation or perturbation of cadherin function.     

Cadherin-Mediated Adhesion Is Required for Normal Growth Regulation of Human Gingival Epithelial Cells

The cadherins are a family of cell membrane proteins that mediate calcium-dependent cell-cell adhesion. E-cadherin is required for the formation, differentiation, polarization and stratification of epithelia; P-cadherin is also expressed on many epithelia. We report here the first study of cadherin expression in immortalized human gingival epithelial cells (IHGK) and examine the role of cadherins in growth regulation of these cells. We found that the IHGK cells are similar to normal gingival epithelial cells in their cadherin expression and density-dependent inhibition of growth.

The IHGK cells proliferate more rapidly at low calcium concentration (0.15 mM) than at physiological concentrations of calcium (1.8 mM) and magnesium (0.65 mM; Ca/Mg medium) suggesting that calcium is required for density-dependent regulation of proliferation. To evaluate the possibility that cadherin function is required for contact inhibition in these cells, we grew them in Ca/Mg medium in the presence of adhesion-blocking anti-cadherin monoclonal antibodies. At anti-E-cadherin concentrations sufficient to disrupt cell-cell adhesion, the proliferation of the IHGK cells was similar to that observed in medium containing 0.2 mM EDTA. Anti-P-cadherin had a much weaker effect on cell proliferation than anti-E-cadherin, and cells grown in medium containing both antibodies grew at intermediate rates. The increased proliferation of the IHGK cells in either low calcium medium or Ca/Mg medium containing adhesion-blocking anti-cadherin antibodies suggests that cadherin-medi-ated adhesion is required for density-dependent regulation of growth of these cells.     

Plasticity of cadherin-catenin expression in the melanocyte lineage

Cadherins are calcium-dependent cell adhesion receptors with strong morphoregulatory functions. To mediate functional adhesion, cadherins must interact with actin cytoskeleton. Catenins are cytoplasmic proteins that mediate the interactions between cadherins and the cytoskeleton. In addition to their role in cell-cell adhesion, catenins also participate in signaling pathways that regulate cell growth and differentiation. Cadherins and catenins appear to be involved in melanocyte development and transformation. Here, we investigated the function of cadherin-catenin complexes in the normal development and transformation of melanocytes by studying the patterns of expression of the cell-cell adhesion molecules, E-, N- and P-cadherin, and the expression of their cytoplasmic partners, alpha-, beta- and gamma-catenin during murine development. Similar analyses were performed in vitro using murine melanoblast, melanocyte, and melanoma cell lines in the presence and absence of keratinocytes, the cells with which melanocytes interact in vivo. Overall, the results suggest that the expression of cadherins and catenins is very plastic and depends on their environment as well as the transformation status of the cells. This plasticity is important in fundamental cellular mechanisms associated with normal and pathological ontogenesis, as well as with tumorigenesis.     

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.     

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