Neural crest motility and integrin regulation are distinct in cranial and trunk populations

Developmentally regulated expression of integrin alpha-6 distinguishes neural crest derivatives in the skin

Neural crest-derived cells play essential roles in skin function and homeostasis. However, how they interact with environmental cues and differentiate into functional skin cells remains unclear. Using a combination of single-cell data analysis, neural crest lineage tracing, and flow cytometry, we found that the expression of integrin α6 (ITGA6) in neural crest and its derivatives was developmentally regulated and that ITGA6 could serve as a functional surface marker for distinguishing neural crest derivatives in the skin. Based on the expression of ITGA6, Wnt1-Cre lineage neural crest derivatives in the skin could be categorized into three subpopulations, namely, ITGA6^bright, ITGA6^dim, and ITGA6^neg, which were found to be Schwann cells, melanocytes, and fibroblasts, respectively. We further analyzed the signature genes and transcription factors that specifically enriched in each cell subpopulation, as well as the ligand or receptor molecules, mediating the potential interaction with other cells of the skin. Additionally, we found that Hmx1 and Lhx8 are specifically expressed in neural crest-derived fibroblasts, while Zic1 and homeobox family genes are expressed in mesoderm-derived fibroblasts, indicating the distinct development pathways of fibroblasts of different origins. Our study provides insights into the regulatory landscape of neural crest cell development and identifies potential markers that facilitate the isolation of different neural crest derivatives in the skin.

Hox proteins as regulators of extracellular matrix interactions during neural crest migration

Emerging during embryogenesis, the neural crest are a migratory, transient population of multipotent stem cell that differentiates into various cell types in vertebrates. Neural crest cells arise along the anterior-posterior extent of the neural tube, delaminate and migrate along routes to their final destinations. The factors that orchestrate how neural crest cells undergo delamination and their subsequent sustained migration is not fully understood. This review provides a primer about neural crest epithelial-to-mesenchymal transition (EMT), with a special emphasis on the role of the Extracellular matrix (ECM), cellular effector proteins of EMT, and subsequent migration. We also summarize published findings that link the expression of Hox transcription factors to EMT and ECM modification, thereby implicating Hox factors in regulation of EMT and ECM remodeling during neural crest cell ontogenesis.

Dynamic integration of enteric neural stem cells in ex vivo organotypic colon cultures

Enteric neural stem cells (ENSC) have been identified as a possible treatment for enteric neuropathies. After in vivo transplantation, ENSC and their derivatives have been shown to engraft within colonic tissue, migrate and populate endogenous ganglia, and functionally integrate with the enteric nervous system. However, the mechanisms underlying the integration of donor ENSC, in recipient tissues, remain unclear. Therefore, we aimed to examine ENSC integration using an adapted ex vivo organotypic culture system. Donor ENSC were obtained from Wnt1^cre/+;R26R^YFP/YFP mice allowing specific labelling, selection and fate-mapping of cells. YFP⁺ neurospheres were transplanted to C57BL6/J (6–8-week-old) colonic tissue and maintained in organotypic culture for up to 21 days. We analysed and quantified donor cell integration within recipient tissues at 7, 14 and 21 days, along with assessing the structural and molecular consequences of ENSC integration. We found that organotypically cultured tissues were well preserved up to 21-days in ex vivo culture, which allowed for assessment of donor cell integration after transplantation. Donor ENSC-derived cells integrated across the colonic wall in a dynamic fashion, across a three-week period. Following transplantation, donor cells displayed two integrative patterns; longitudinal migration and medial invasion which allowed donor cells to populate colonic tissue. Moreover, significant remodelling of the intestinal ECM and musculature occurred upon transplantation, to facilitate donor cell integration within endogenous enteric ganglia. These results provide critical evidence on the timescale and mechanisms, which regulate donor ENSC integration, within recipient gut tissue, which are important considerations in the future clinical translation of stem cell therapies for enteric disease.

Alginate Microencapsulation for Three-Dimensional In Vitro Cell Culture

Advances in microscale 3D cell culture systems have helped to elucidate cellular physiology, understand mechanisms of stem cell differentiation, produce pathophysiological models, and reveal important cell-cell and cell-matrix interactions. An important consideration for such studies is the choice of material for encapsulating cells and associated extracellular matrix (ECM). This Review focuses on the use of alginate hydrogels, which are versatile owing to their simple gelation process following an ionic cross-linking mechanism in situ, with no need for procedures that can be potentially toxic to cells, such as heating, the use of solvents, and UV exposure. This Review aims to give some perspectives, particularly to researchers who typically work more with poly(dimethylsiloxane) (PDMS), on the use of alginate as an alternative material to construct microphysiological cell culture systems. More specifically, this Review describes how physicochemical characteristics of alginate hydrogels can be tuned with regards to their biocompatibility, porosity, mechanical strength, ligand presentation, and biodegradability. A number of cell culture applications are also described, and these are subcategorized according to whether the alginate material is used to homogeneously embed cells, to micropattern multiple cellular microenvironments, or to provide an outer shell that creates a space in the core for cells and other ECM components. The Review ends with perspectives on future challenges and opportunities for 3D cell culture applications.

The road best traveled: Neural crest migration upon the extracellular matrix

Neural crest cells have the extraordinary task of building much of the vertebrate body plan, including the craniofacial cartilage and skeleton, melanocytes, portions of the heart, and the peripheral nervous system. To execute these developmental programs, stationary premigratory neural crest cells first acquire the capacity to migrate through an extensive process known as the epithelial-to-mesenchymal transition. Once motile, neural crest cells must traverse a complex environment consisting of other cells and the protein-rich extracellular matrix in order to get to their final destinations. Herein, we will highlight some of the main molecular machinery that allow neural crest cells to first exit the neuroepithelium and then later successfully navigate this intricate in vivo milieu. Collectively, these extracellular and intracellular factors mediate the appropriate migration of neural crest cells and allow for the proper development of the vertebrate embryo.

Re-engineered cell-derived extracellular matrix as a new approach to clarify the role of native ECM

An extracellular matrix (ECM) has both biochemical and mechanophysical characteristics obtained from multiple components, which provides cells a dynamic microenvironment. During reciprocal interactions with ECM, the cells actively remodel the matrix, including synthesis, degradation, and chemical modification, which play a pivotal role in various biological events such as disease progression or tissue developmental processes. Since a cell-derived decellularized ECM (cdECM) holds in vivo-like compositional heterogeneity and interconnected fibrillary architecture, it has received much attention as a promising tool for developing more physiological in vitro model systems. Despite these advantages, the cdECM has obvious limitations to mimic versatile ECMs precisely, suggesting the need for improved in vitro modeling to clarify the functions of native ECM. Recent studies propose to tailor the cdECM via biochemically, biomechanically, or incorporation with other systems as a new approach to address the limitations. In this chapter, we summarize the studies that re-engineered the cdECM to examine the features of native ECM in-depth and to increase physiological relevancy.

Regulation of white and brown adipocyte differentiation by RhoGAP DLC1

Adipose tissues constitute an important component of metabolism, the dysfunction of which can cause obesity and type II diabetes. Here we show that differentiation of white and brown adipocytes requires Deleted in Liver Cancer 1 (DLC1), a Rho GTPase Activating Protein (RhoGAP) previously studied for its function in liver cancer. We identified Dlc1 as a super-enhancer associated gene in both white and brown adipocytes through analyzing the genome-wide binding profiles of PPARγ, the master regulator of adipogenesis. We further observed that Dlc1 expression increases during differentiation, and knockdown of Dlc1 by siRNA in white adipocytes reduces the formation of lipid droplets and the expression of fat marker genes. Moreover, knockdown of Dlc1 in brown adipocytes reduces expression of brown fat-specific genes and diminishes mitochondrial respiration. Dlc1-/- knockout mouse embryonic fibroblasts show a complete inability to differentiate into adipocytes, but this phenotype can be rescued by inhibitors of Rho-associated kinase (ROCK) and filamentous actin (F-actin), suggesting the involvement of Rho pathway in DLC1-regulated adipocyte differentiation. Furthermore, PPARγ binds to the promoter of Dlc1 gene to regulate its expression during both white and brown adipocyte differentiation. These results identify DLC1 as an activator of white and brown adipocyte differentiation, and provide a molecular link between PPARγ and Rho pathways.

Construction and Characterization of Recombinant HEK Cell Over Expressing α4 Integrin

PURPOSE

Integrins are heterodimeric membrane proteins, which are exposed to post translational modifications in eukaryotic cells in contrast to prokaryotic cells. These modifications provide advantages for production of proper nanobody, mono and polyclonal antibody against this surface protein and also in aptamer selection process. Since the majority of diagnostic and therapeutic antibodies, target the surface epitopes, eukaryotic membrane proteins provide an appropriate model for further investigation on therapeutic agents.

METHODS

Escherichia coli strain top 10, was used as host for ITGA-4 expression vector encoding the human integrin α4. The plasmid was extracted and consequently, ITGA-4 vector was digested to make a linear plasmid. Human Embryonic Kidney-293 (HEK-293) cell transfected with linear plasmid and subsequently screened for stable ITGA-4 expressing Cells. Three separated clones were isolated twenty one days after transfection. Chromosomal DNA was extracted from ITGA-4-transfected cells. The presence of ITGA-4 gene in HEK-293 genome was confirmed by PCR. The expression level of ITGA-4 on HEK-293 cells was also analyzed by Flow cytometry.

RESULTS

Flow cytometric analysis showed that HEK-293 cells have no expression of integrin α4 on their surface while 95% of transfected HEK-293 cells with ITGA4, expressed different levels of integrin α4 on their surfaces which correlates well with genomic DNA PCR amplification results.

CONCLUSION

The results suggest that we have successfully constructed the integrin α4 expressing HEK293 cell, which will facilitate further research into the production of antibody, nanobody and aptamer against α4 integrin.

Neural crest: The fourth germ layer

The neural crest cells (NCCs), a transient group of cells that emerges from the dorsal aspect of the neural tube during early vertebrate development has been a fascinating group of cells because of its multipotency, long range migration through embryo and its capacity to generate a prodigious number of differentiated cell types. For these reasons, although derived from the ectoderm, the neural crest (NC) has been called the fourth germ layer. The non neural ectoderm, the neural plate and the underlying mesoderm are needed for the induction and formation of NC cells. Once formed, NC cells start migrating as a wave of cells, moving away from the neuroepithelium and quickly splitting into distinct streams. These migrating NCCs home in to different regions and give rise to plethora of tissues. Umpteen number of signaling molecules are essential for formation, epithelial mesenchymal transition, delamination, migration and localization of NCC. Authors believe that a clear understanding of steps and signals involved in NC formation, migration, etc., may help in understanding the pathogenesis behind cancer metastasis and many other diseases. Hence, we have taken this review to discuss the various aspects of the NC cells.

Deletion of integrin-linked kinase from neural crest cells in mice results in aortic aneurysms and embryonic lethality

Neural crest cells (NCCs) participate in the remodeling of the cardiac outflow tract and pharyngeal arch arteries during cardiovascular development. Integrin-linked kinase (ILK) is a serine/threonine kinase and a major regulator of integrin signaling. It links integrins to the actin cytoskeleton and recruits other adaptor molecules into a large complex to regulate actin dynamics and integrin function. Using the Cre-lox system, we deleted Ilk from NCCs of mice to investigate its role in NCC morphogenesis. The resulting mutants developed a severe aneurysmal arterial trunk that resulted in embryonic lethality during late gestation. Ilk mutants showed normal cardiac NCC migration but reduced differentiation into smooth muscle within the aortic arch arteries and the outflow tract. Within the conotruncal cushions, Ilk-deficient NCCs exhibited disorganization of F-actin stress fibers and a significantly rounder morphology, with shorter cellular projections. Additionally, absence of ILK resulted in reduced in vivo phosphorylation of Smad3 in NCCs, which correlated with reduced αSMA levels. Our findings resemble those seen in Pinch1 and β1 integrin conditional mutant mice, and therefore support that, in neural crest-derived cells, ILK and Pinch1 act as cytoplasmic effectors of β1 integrin in a pathway that protects against aneurysms. In addition, our conditional Ilk mutant mice might prove useful as a model to study aortic aneurysms caused by reduced Smad3 signaling, as occurs in the newly described aneurysms-osteoarthritis syndrome, for example.

Follow-the-leader cell migration requires biased cell-cell contact and local microenvironmental signals

Directed cell migration often involves at least two types of cell motility that include multicellular streaming and chain migration. However, what is unclear is how cell contact dynamics and the distinct microenvironments through which cells travel influence the selection of one migratory mode or the other. The embryonic and highly invasive neural crest (NC) are an excellent model system to study this question since NC cells have been observed in vivo to display both of these types of cell motility. Here, we present data from tissue transplantation experiments in chick and in silico modeling that test our hypothesis that cell contact dynamics with each other and the microenvironment promote and sustain either multicellular stream or chain migration. We show that when premigratory cranial NC cells (at the pre-otic level) are transplanted into a more caudal region in the head (at the post-otic level), cells alter their characteristic stream behavior and migrate in chains. Similarly, post-otic NC cells migrate in streams after transplantation into the pre-otic hindbrain, suggesting that local microenvironmental signals dictate the mode of NC cell migration. Simulations of an agent-based model (ABM) that integrates the NC cell behavioral data predict that chain migration critically depends on the interplay of biased cell-cell contact and local microenvironment signals. Together, this integrated modeling and experimental approach suggests new experiments and offers a powerful tool to examine mechanisms that underlie complex cell migration patterns.

Human fetal keratocytes have multipotent characteristics in the developing avian embryo

The human cornea contains stem cells that can be induced to express markers consistent with multipotency in cell culture; however, there have been no studies demonstrating that human corneal keratocytes are multipotent. The objective of this study is to examine the potential of human fetal keratocytes (HFKs) to differentiate into neural crest-derived tissues when challenged in an embryonic environment. HFKs were injected bilaterally into the cranial mesenchyme adjacent to the neural tube and the periocular mesenchyme in chick embryos at embryonic days 1.5 and 3, respectively. The injected keratocytes were detected by immunofluorescence using the human cell-specific marker, HuNu. HuNu-positive keratocytes injected along the neural crest pathway were localized adjacent to HNK-1-positive migratory host neural crest cells and in the cardiac cushion mesenchyme. The HuNu-positive cells transformed into neural crest derivatives such as smooth muscle in cranial blood vessels, stromal keratocytes, and corneal endothelium. However, they failed to form neurons despite their presence in the condensing trigeminal ganglion. These results show that HFKs retain the ability to differentiate into some neural crest-derived tissues. Their ability to respond to embryonic cues and generate corneal endothelium and stromal keratocytes provides a basis for understanding the feasibility of creating specialized cells for possible use in regenerative medicine.

Expression and function of cell adhesion molecules during neural crest migration

Neural crest cells are highly migratory cells that give rise to many derivatives including peripheral ganglia, craniofacial structures and melanocytes. Neural crest cells migrate along defined pathways to their target sites, interacting with each other and their environment as they migrate. Cell adhesion molecules are critical during this process. In this review we discuss the expression and function of cell adhesion molecules during the process of neural crest migration, in particular cadherins, integrins, members of the immunoglobulin superfamily of cell adhesion molecules, and the proteolytic enzymes that cleave these cell adhesion molecules. The expression and function of these cell adhesion molecules and proteases are compared across neural crest emigrating from different axial levels, and across different species of vertebrates.

Slits affect the timely migration of neural crest cells via Robo receptor

BACKGROUND

Neural crest cells emerge by delamination from the dorsal neural tube and give rise to various components of the peripheral nervous system in vertebrate embryos. These cells change from non-motile into highly motile cells migrating to distant areas before further differentiation. Mechanisms controlling delamination and subsequent migration of neural crest cells are not fully understood. Slit2, a chemorepellant for axonal guidance that repels and stimulates motility of trunk neural crest cells away from the gut has recently been suggested to be a tumor suppressor molecule. The goal of this study was to further investigate the role of Slit2 in trunk neural crest cell migration by constitutive expression in neural crest cells.

RESULTS

We found that Slit gain-of-function significantly impaired neural crest cell migration while Slit loss-of-function favored migration. In addition, we observed that the distribution of key cytoskeletal markers was disrupted in both gain and loss of function instances.

CONCLUSIONS

These findings suggest that Slit molecules might be involved in the processes that allow neural crest cells to begin migrating and transitioning to a mesenchymal type.

Neural crest cells and motor axons in avians: Common and distinct migratory molecules

It has long been thought that the same molecules guide both trunk neural crest cells and motor axons as these cell types grow and extend to their target regions in developing embryos. There are common territories that are navigated by these cell types: both cells grow through the rostral portion of the somitic sclerotomes and avoid the caudal half of the sclerotomes. However, these cell types seem to use different molecules to guide them to their target regions. In this review, I will talk about the common and distinct methods of migration taken by trunk neural crest cells and motor axons as they grow and populate their target regions through chick embryos at the level of the trunk.

Cell-extracellular matrix versus cell-cell interactions during the development of the cochlear-vestibular ganglion

Cells destined to become the neurones of the cochlear-vestibular ganglion (CVG) originate within the otic epithelium. Early in development they detach from their neighbors and migrate out of the epithelium, where they coalesce to form the CVG. To accomplish this process, the neuroblasts must modify their interactions with other cells within the epithelium and with proteins in the extracellular matrix to allow for repositioning. The aim of this study was to investigate the roles of the major families of adhesion molecules that mediate cellular interactions with the extracellular matrix, the integrins, and with other cells, the cadherins, in neuroblast segregation from the otic epithelium. The expression of classical cadherins increased in migrating neuroblasts compared with the otic epithelium. Quantitative RT-PCR revealed that this was concomitant with down-regulation of E-cadherin and up-regulation of N-cadherin in the migrating cells. In contrast, the level of β1 integrin expression by the epithelium was maintained in migrating neuroblasts. However, although multiple integrin ligands were expressed within the otic basement membrane at this stage of development, only fibronectin (FN) supported neuroblast migration along the substrate in vitro. Inhibition of β1 integrins resulted in significantly reduced linear migration on FN. Importantly, neuroblasts retained the ability to segregate from the epithelium but remained compacted immediately adjacent to the originating tissue, suggesting dominance of cell-cell over cell-matrix interactions. These data suggest that the balance between cell-cell and cell-substratum interactions directs otic neuroblast migration and gangliogenesis.

Fibronectin and integrin alpha 5 play essential roles in the development of the cardiac neural crest

Cardiac neural crest (CNC) plays a requisite role during cardiovascular development and defects in the formation of CNC-derived structures underlie several common forms of human congenital birth defects. Migration of the CNC cells to their destinations as well as expansion and maintenance of these cells are important for the normal development of the cardiac outflow tract and aortic arch arteries; however, molecular mechanisms regulating these processes are not well-understood. Fibronectin (FN) protein is present along neural crest migration paths and neural crest cells migrate when plated on FN in vitro; therefore, we tested the role of FN during the development of the CNC in vivo. Our analysis of the fate of the neural crest shows that CNC cells reach their destinations in the branchial arches and the cardiac outflow tract in the absence of FN or its cellular receptor integrin α5β1. However, we found that FN and integrin α5 modulate CNC proliferation and survival, and are required for the presence of normal numbers of CNC cells at their destinations.

The extracellular matrix in development and morphogenesis: a dynamic view

The extracellular matrix (ECM) is synthesized and secreted by embryonic cells beginning at the earliest stages of development. Our understanding of ECM composition, structure and function has grown considerably in the last several decades and this knowledge has revealed that the extracellular microenvironment is critically important for cell growth, survival, differentiation and morphogenesis. ECM and the cellular receptors that interact with it mediate both physical linkages with the cytoskeleton and the bidirectional flow of information between the extracellular and intracellular compartments. This review considers the range of cell and tissue functions attributed to ECM molecules and summarizes recent findings specific to key developmental processes. The importance of ECM as a dynamic repository for growth factors is highlighted along with more recent studies implicating the 3-dimensional organization and physical properties of the ECM as it relates to cell signaling and the regulation of morphogenetic cell behaviors. Embryonic cell and tissue generated forces and mechanical signals arising from ECM adhesion represent emerging areas of interest in this field.

Cranial neural crest migration: new rules for an old road

The neural crest serve as an excellent model to better understand mechanisms of embryonic cell migration. Cell tracing studies have shown that cranial neural crest cells (CNCCs) emerge from the dorsal neural tube in a rostrocaudal manner and are spatially distributed along stereotypical, long distance migratory routes to precise targets in the head and branchial arches. Although the CNCC migratory pattern is a beautifully choreographed and programmed invasion, the underlying orchestration of molecular events is not well known. For example, it is still unclear how single CNCCs react to signals that direct their choice of direction and how groups of CNCCs coordinate their interactions to arrive at a target in an ordered manner. In this review, we discuss recent cellular and molecular discoveries of the CNCC migratory pattern. We focus on events from the time when CNCCs encounter the tissue adjacent to the neural tube and their travel through different microenvironments and into the branchial arches. We describe the patterning of discrete cell migratory streams that emerge from the hindbrain, rhombomere (r) segments r1-r7, and the signals that coordinate directed migration. We propose a model that attempts to unify many complex events that establish the CNCC migratory pattern, and based on this model we integrate information between cranial and trunk neural crest development.

Identification and characterization of Dlc1 isoforms in the mouse and study of the biological function of a single gene trapped isoform

Background

The Dlc1 (deleted in liver cancer 1) tumour suppressor gene codes for a RhoGTPase activating protein that is found inactivated in many tumour types. Several transcriptional isoforms have been described but the functional significance and tissue distribution of each form is presently poorly understood. Also, differences in the number of isoforms and splice variants reported still exist between different mammalian species. In order to better understand the number and function of the different variants of the Dlc1 gene in the mouse, we have carried out a detailed analysis. Extensive 3' RACE experiments were carried out in order to identify all possible Dlc1 isoforms and splice variants in the mouse. In addition, we have generated a gene trapped mouse that targets one of these isoforms in order to study its biological function. The effect of this gene trap insertion on the splicing of other isoforms has also been studied.

Results

In addition to the known 6.1 and 6.2 Kb transcripts of Dlc1, our study revealed the existence of a novel 7.6 Kb transcriptional isoform in the mouse, which corresponds to the human 7.4 Kb (KIAA1723) cDNA transcript. A gene trapped embryonic cell line, with an insertion between Exon 1 and 2 of the 6.1 Kb transcriptional isoform, was used to generate a transgenic mouse. This line showed a significant reduction in the expression of the trapped isoform. However, reduced expression of the other isoforms was not seen. Mice heterozygous for the gene trapped allele were phenotypically normal, but homozygous mutant embryos did not survive beyond 10.5 days post coitum. Dlc1^gt/gt embryos showed defects in the brain, heart, and placental blood vessels. Cultured serum-free mouse embryo cells from Dlc1 deficient embryos had elevated RhoA activity and displayed alterations in the organization of actin filaments and focal adhesions. The Dlc1 deficient cells also exhibited increased wound closure in an in vitro scratch assay.

Conclusions

The mouse has three major transcriptional isoforms of the Dlc1 gene that are differentially expressed in various tissues. A mouse with exon 1 of the 6.1 Kb transcript gt resulted in hypomorphic expression of Dlc1 protein and an embryonic lethal phenotype in the homozygous condition, which indicates that this isoform plays a major role in mouse development. The Dlc1 deficient cells showed altered cytoskeleton structure, increased RhoA activity and cellular migration.

The developmental roles of the extracellular matrix: beyond structure to regulation

Cells in multicellular organisms are surrounded by a complex three-dimensional macromolecular extracellular matrix (ECM). This matrix, traditionally thought to serve a structural function providing support and strength to cells within tissues, is increasingly being recognized as having pleiotropic effects in development and growth. Elucidation of the role that the ECM plays in developmental processes has been significantly advanced by studying the phenotypic and developmental consequences of specific genetic alterations of ECM components in the mouse. These studies have revealed the enormous contribution of the ECM to the regulation of key processes in morphogenesis and organogenesis, such as cell adhesion, proliferation, specification, migration, survival, and differentiation. The ECM interacts with signaling molecules and morphogens thereby modulating their activities. This review considers these advances in our understanding of the function of ECM proteins during development, extending beyond their structural capacity, to embrace their new roles in intercellular signaling.

Focal adhesion kinase is required for neural crest cell morphogenesis during mouse cardiovascular development

Neural crest cells (NCCs) participate in the remodeling of the cardiac outflow tract and pharyngeal arch arteries during cardiovascular development. Focal adhesion kinase (FAK) mediates signal transduction by integrin and growth factor receptors, each of which is important for normal cardiovascular development. To investigate the role of FAK in NCC morphogenesis, we deleted it in murine NCCs using Wnt1cre, yielding craniofacial and cardiovascular malformations resembling those observed in individuals with DiGeorge syndrome. In these mice, we observed normal cardiac NCC migration but reduced differentiation into smooth muscle within the aortic arch arteries and impaired cardiac outflow tract rotation, which resulted in a dextroposed aortic root. Moreover, within the conotruncal cushions, Fak-deficient NCCs formed a less organized mesenchyme, with reduced expression of perlecan and semaphorin 3C, and exhibited disorganized F-actin stress fibers within the aorticopulmonary septum. Additionally, absence of Fak resulted in reduced in vivo phosphorylation of Crkl and Erk1/2, components of a signaling pathway essential for NCC development. Consistent with this, both TGF-beta and FGF induced FAK and Crkl phosphorylation in control but not Fak-deficient NCCs in vitro. Our results indicate that FAK plays an essential role in cardiac outflow tract development by promoting the activation of molecules such as Crkl and Erk1/2.

The retinoic acid inducible Cas-family signaling protein Nedd9 regulates neural crest cell migration by modulating adhesion and actin dynamics

Cell migration is essential for the development of numerous structures derived from embryonic neural crest cells (NCCs), however the underlying molecular mechanisms are incompletely understood. NCCs migrate long distances in the embryo and contribute to many different cell types, including peripheral neurons, glia and pigment cells. In the present work we report expression of Nedd9, a scaffolding protein within the integrin signaling pathway, in non-lineage-restricted neural crest progenitor cells. In particular, Nedd9 was found to be expressed in the dorsal neural tube at the time of neural crest delamination and in early migrating NCCs. To analyze the role of Nedd9 in neural crest development we performed loss- and gain-of-function experiments and examined the subsequent effects on delamination and migration in vitro and in vivo. Our results demonstrate that loss of Nedd9 activity in chick NCCs perturbs cell spreading and the density of focal complexes and actin filaments, properties known to depend on integrins. Moreover, a siRNA dose-dependent decrease in Nedd9 activity results in a graded reduction of NCC's migratory distance while forced overexpression increases it. Retinoic acid (RA) was found to regulate Nedd9 expression in NCCs. Our results demonstrate in vivo that Nedd9 promotes the migration of NCCs in a graded manner and suggest a role for RA in the control of Nedd9 expression levels.

Fibronectin promotes differentiation of neural crest progenitors endowed with smooth muscle cell potential

The neural crest (NC) is a model system used to investigate multipotency during vertebrate development. Environmental factors control NC cell fate decisions. Despite the well-known influence of extracellular matrix molecules in NC cell migration, the issue of whether they also influence NC cell differentiation has not been addressed at the single cell level. By analyzing mass and clonal cultures of mouse cephalic and quail trunk NC cells, we show for the first time that fibronectin (FN) promotes differentiation into the smooth muscle cell phenotype without affecting differentiation into glia, neurons, and melanocytes. Time course analysis indicated that the FN-induced effect was not related to massive cell death or proliferation of smooth muscle cells. Finally, by comparing clonal cultures of quail trunk NC cells grown on FN and collagen type IV (CLIV), we found that FN strongly increased both NC cell survival and the proportion of unipotent and oligopotent NC progenitors endowed with smooth muscle potential. In contrast, melanocytic progenitors were prominent in clonogenic NC cells grown on CLIV. Taken together, these results show that FN promotes NC cell differentiation along the smooth muscle lineage, and therefore plays an important role in fate decisions of NC progenitor cells.

Connexons and cell adhesion: a romantic phase

Recent evidence indicates, that gap junction forming proteins do not only contribute to intercellular communication (Kanno and Saffitz in Cardiovasc Pathol 10:169–177, 2001; Saez et al. in Physiol Rev 83:1359–1400, 2003), ion homeostasis and volume control (Goldberg et al. in J Biol Chem 277:36725–36730, 2002; Saez et al. in Physiol Rev 83:1359–1400, 2003). They also serve biological functions in a mechanical sense, supporting adherent connections between neighbouring cells of epithelial and non-epithelial tissues (Clair et al. in Exp Cell Res 314:1250–1265, 2008; Shaw et al. in Cell 128:547–560, 2007), where they stabilize migratory pathways in the developing central nervous system (Elias et al. in Nature 448:901–907, 2007; Malatesta et al. in Development 127:5253–5263, 2000; Noctor et al. in Nature 409:714–720, 2001; Rakic in Brain Res 33:471–476, 1971; J Comp Neurol 145:61–83 1972; Science 241:170–176, 1988), or mediate polarized movements and directionality of neural crest cells during organogenesis (Kirby and Waldo in Circ Res 77:211–215, 1995; Xu et al. in Development 133:3629–3639, 2006). Since, most data describing adhesive properties of gap junctions delt with connexin 43 (Cx43) (Beardslee et al. in Circ Res 83:629–635, 1998), we will focus our brief review on this isoform.

Neural crest motility on fibronectin is regulated by integrin activation

Cell migration is essential for proper development of numerous structures derived from embryonic neural crest cells (NCCs). Although recent work has shown that receptor recycling plays an important role in NCC motility on laminin, the molecular mechanisms regulating NCC motility on fibronectin remain unclear. One mechanism by which cells regulate motility is by modulating the affinity of integrin receptors. Here, we provide evidence that cranial and trunk NCCs rely on functional regulation of integrins to migrate efficiently on fibronectin (FN) in vitro. For NCCs cultured on fibronectin, velocity decreases after Mn2+ application (a treatment that activates all surface integrins) while velocity on laminin (LM) is not affected. The distribution of activated integrin beta 1 receptors on the surface of NCCs is also substratum-dependent. Integrin activation affects cranial and trunk NCCs differently when cultured on different concentrations of FN substrata; only cranial NCCs slow in a FN concentration-dependent manner. Furthermore, Mn2+ treatment alters the distribution and number of activated integrin beta 1 receptors on the surface of cranial and trunk NCCs in different ways. We provide a hypothesis whereby a combination of activated surface integrin levels and the degree to which those receptors are clustered determines NCC motility on fibronectin.

The role of the integrin LFA-1 in T-lymphocyte migration

A successful immune response depends on the migration of lymphocytes into lymph nodes or inflamed tissues where they make contact with antigen-presenting cells. We are interested in how one member of the integrin family, leukocyte function-associated antigen-1 (LFA-1), controls the function and, in particular, the migration of immune cells. We find that this integrin operates not only as an adhesion receptor for T lymphoblasts (T cells) but also induces their migration in vitro at approximately 15 microm/min. Migration requires active myosin light chain kinase at the leading edge and Rho kinase at the trailing edge of the cell. Two active conformations of LFA-1 are differently distributed on the T-cell membrane and regulate independent aspects of migration. High-affinity LFA-1 is located in a midcell 'focal zone' and influences the speed of migration, whereas intermediate affinity LFA-1 controls leading edge adhesions. Manipulating LFA-1 conformation in vivo can be performed, for example, by creating the active conformation in a transgenic mouse, and this model gives further insight into the role of LFA-1 in migration. In humans, the beneficial effect of functioning CD18 integrins in combating infections in vivo is illustrated by rare patients displaying two forms of leukocyte adhesion deficiency. In summary, we speculate that T cells have evolved a mode of rapid migration that is of paramount importance in achieving the high-speed immune surveillance upon which depends the body's protection against diverse invaders from pathogens to cancer cells.

Role of integrins in peripheral nerves and hereditary neuropathies

Interactions between Schwann cells and extracellular matrix on one surface, and axons on the other, are required for correct myelination in the developing peripheral nervous system. Integrins are transmembrane proteins that mediate the former in association with other surface receptors. This review focuses on the role that integrins play in the development of the peripheral nervous system, and in inherited human peripheral neuropathies. Here we describe recent findings on integrin signaling to different intracellular pathways, focusing on cell adhesion, migration, and polarization. Then we use information derived from recent experiments of targeted mutagenesis in mice to show that, consistent with temporally regulated expression, different integrins serve multiple roles in developing nerve.

Connexin 43-mediated modulation of polarized cell movement and the directional migration of cardiac neural crest cells

Connexin 43 knockout (Cx43α1KO) mice have conotruncal heart defects that are associated with a reduction in the abundance of cardiac neural crest cells (CNCs) targeted to the heart. In this study, we show CNCs can respond to changing fibronectin matrix density by adjusting their migratory behavior,with directionality increasing and speed decreasing with increasing fibronectin density. However, compared with wild-type CNCs, Cx43α1KO CNCs show reduced directionality and speed, while CNCs overexpressing Cx43α1 from the CMV43 transgenic mice show increased directionality and speed. Altered integrin signaling was indicated by changes in the distribution of vinculin containing focal contacts, and altered temporal response of Cx43α1KO and CMV43 CNCs to β1 integrin function blocking antibody treatment. High resolution motion analysis showed Cx43α1KO CNCs have increased cell protrusive activity accompanied by the loss of polarized cell movement. They exhibited an unusual polygonal arrangement of actin stress fibers that indicated a profound change in cytoskeletal organization. Semaphorin 3A, a chemorepellent known to inhibit integrin activation, was found to inhibit CNC motility, but in the Cx43α1KO and CMV43 CNCs, cell processes failed to retract with semaphorin 3A treatment. Immunohistochemical and biochemical analyses suggested close interactions between Cx43α1,vinculin and other actin-binding proteins. However, dye coupling analysis showed no correlation between gap junction communication level and fibronectin plating density. Overall, these findings indicate Cx43α1 may have a novel function in mediating crosstalk with cell signaling pathways that regulate polarized cell movement essential for the directional migration of CNCs.

Dynamic partitioning into lipid rafts controls the endo-exocytic cycle of the alphaL/beta2 integrin, LFA-1, during leukocyte chemotaxis

Cell migration entails the dynamic redistribution of adhesion receptors from the cell rear toward the cell front, where they form new protrusions and adhesions. This process may involve regulated endo-exocytosis of integrins. Here we show that in primary neutrophils unengaged alphaL/beta2 integrin (LFA-1) is internalized and rapidly recycled upon chemoattractant stimulation via a clathrin-independent, cholesterol-sensitive pathway involving dynamic partitioning into detergent-resistant membranes (DRM). Persistent DRM association is required for recycling of the internalized receptor because 1) >90% of endocytosed LFA-1 is associated with DRM, and a large fraction of the internalized receptor colocalizes intracellularly with markers of DRM and the recycling endocytic compartment; 2) a recycling-defective mutant (alphaL/beta2Y735A) dissociates rapidly from DRM upon being endocytosed and is subsequently diverted into a late endosomal pathway; and 3) a dominant negative Rab11 mutant (Rab11S25N) induces intracellular accumulation of endocytosed alphaL/beta2 and prevents its enrichment in chemoattractant-induced lamellipodia. Notably, chemokine-induced migration of neutrophils over immobilized ICAM-1 is abrogated by cholesterol-sequestering agents. We propose that DRM-associated endocytosis allows efficient retrieval of integrins, as they detach from their ligands, followed by polarized recycling to areas of the plasma membrane, such as lamellipodia, where they establish new adhesive interactions and promote outside-in signaling events.

Selective modulation of integrin-mediated cell migration by distinct ADAM family members

A disintegrin and a metalloprotease (ADAM) family members have been implicated in many biological processes. Although it is recognized that recombinant ADAM disintegrin domains can interact with integrins, little is known about ADAM-integrin interactions in cellular context. Here, we tested whether ADAMs can selectively regulate integrin-mediated cell migration. ADAMs were expressed in Chinese hamster ovary cells that express defined integrins (alpha4beta1, alpha5beta1, or both), and cell migration on full-length fibronectin or on its alpha4beta1 or alpha5beta1 binding fragments was studied. We found that ADAMs inhibit integrin-mediated cell migration in patterns dictated by the integrin binding profiles of their isolated disintegrin domains. ADAM12 inhibited cell migration mediated by the alpha4beta1 but not the alpha5beta1 integrin. ADAM17 had the reciprocal effect; it inhibited alpha5beta1- but not alpha4beta1-mediated cell migration. ADAM19 and ADAM33 inhibited migration mediated by both alpha4beta1 and alpha5beta1 integrins. A point mutation in the ADAM12 disintegrin loop partially reduced the inhibitory effect of ADAM12 on cell migration on the alpha4beta1 binding fragment of fibronectin, whereas mutations that block metalloprotease activity had no effect. Our results indicate that distinct ADAMs can modulate cell migration mediated by specific integrins in a pattern dictated, at least in part, by their disintegrin domains.

A talin-dependent LFA-1 focal zone is formed by rapidly migrating T lymphocytes

Cells such as fibroblasts and endothelial cells migrate through the coordinated responses of discrete integrin-containing focal adhesions and complexes. In contrast, little is known about the organization of integrins on the highly motile T lymphocyte. We have investigated the distribution, activity, and cytoskeletal linkage of the integrin lymphocyte function associated antigen-1 (LFA-1) on human T lymphocytes migrating on endothelial cells and on ligand intercellular adhesion molecule-1 (ICAM-1). The pattern of total LFA-1 varies from low expression in the lamellipodia to high expression in the uropod. However, high affinity, clustered LFA-1 is restricted to a mid-cell zone that remains stable over time and over a range of ICAM-1 densities. Talin is essential for the stability and formation of the LFA-1 zone. Disruption of the talin–integrin link leads to loss of zone integrity and a substantial decrease in speed of migration on ICAM-1. This adhesive structure, which differs from the previously described integrin-containing attachments displayed by many other cell types, we have termed the “focal zone.”

Cranial neural crest recycle surface integrins in a substratum-dependent manner to promote rapid motility

Cell migration is essential for proper development of numerous structures derived from embryonic neural crest cells (NCCs). Although the migratory pathways of NCCs have been determined, the molecular mechanisms regulating NCC motility remain unclear. NCC migration is integrin dependent, and recent work has shown that surface expression levels of particular integrin alpha subunits are important determinants of NCC motility in vitro. Here, we provide evidence that rapid cranial NCC motility on laminin requires integrin recycling. NCCs showed both ligand- and receptor-specific integrin regulation in vitro. On laminin, NCCs accumulated internalized laminin but not fibronectin receptors over 20 min, whereas on fibronectin neither type of receptor accumulated internally beyond 2 min. Internalized laminin receptors colocalized with receptor recycling vesicles and were subsequently recycled back to the cell surface. Blocking receptor recycling with bafilomycin A inhibited NCC motility on laminin, indicating that substratum-dependent integrin recycling is essential for rapid cranial neural crest migration.

Hoxb1 functions in both motoneurons and in tissues of the periphery to establish and maintain the proper neuronal circuitry

Formation of neuronal circuits in the head requires the coordinated development of neurons within the central nervous system (CNS) and neural crest-derived peripheral target tissues. Hoxb1, which is expressed throughout rhombomere 4 (r4), has been shown to be required for the specification of facial branchiomotor neuron progenitors that are programmed to innervate the muscles of facial expression. In this study, we have uncovered additional roles for Hoxb1-expressing cells in the formation and maintenance of the VIIth cranial nerve circuitry. By conditionally deleting the Hoxb1 locus in neural crest, we demonstrate that Hoxb1 is also required in r4-derived neural crest to facilitate and maintain formation of the VIIth nerve circuitry. Genetic lineage analysis revealed that a significant population of r4-derived neural crest is fated to generate glia that myelinate the VIIth cranial nerve. Neural crest cultures show that the absence of Hoxb1 function does not appear to affect overall glial progenitor specification, suggesting that a later glial function is critical for maintenance of the VIIth nerve. Taken together, these results suggest that the molecular program governing the development and maintenance of the VIIth cranial nerve is dependent upon Hoxb1, both in the neural crest-derived glia and in the facial branchiomotor neurons.

Homocysteine inhibits cardiac neural crest cell formation and morphogenesis in vivo

Elevated homocysteine increases the risk of neurocristopathies. Here, we determined whether elevating homocysteine altered the proliferation or number of chick neural crest cells that form between the midotic and third somite in vivo. Homocysteine increased the number of neural tube cells but decreased neural crest cell number. However, the sum total of cells was not different from controls. In controls, the 5-bromo-2'-deoxyuridine-labeling index was higher in newly formed neural crest cells than in their progenitors, paralleling reports showing these progenitors must pass the restriction point before undergoing epithelial-mesenchymal transition. Homocysteine decreased the labeling index of newly formed neural crest cells, suggesting that it inhibited cell cycle progression of neural crest progenitors or the S-phase entry of newly formed neural crest cells. Homocysteine also inhibited neural crest dispersal and decreased the distance they migrated from the neural tube. These results show neural crest morphogenesis is directly altered by elevated homocysteine in vivo. Developmental Dynamics 229:63-73, 2004.