R-cadherin: a novel Ca(2+)-dependent cell-cell adhesion molecule expressed in the retina

Cardiomyopathy related desmocollin-2 prodomain variants affect the intracellular cadherin transport and processing

Background

Arrhythmogenic cardiomyopathy can be caused by genetic variants in desmosomal cadherins. Since cardiac desmosomal cadherins are crucial for cell-cell-adhesion, their correct localization at the plasma membrane is essential.

Methods

Nine desmocollin-2 variants at five positions from various public genetic databases (p.D30N, p.V52A/I, p.G77V/D/S, p.V79G, p.I96V/T) and three additional conserved positions (p.C32, p.C57, p.F71) within the prodomain were investigated in vitro using confocal microscopy. Model variants (p.C32A/S, p.V52G/L, p.C57A/S, p.F71Y/A/S, p.V79A/I/L, p.I96l/A) were generated to investigate the impact of specific amino acids.

Results

We revealed that all analyzed positions in the prodomain are critical for the intracellular transport. However, the variants p.D30N, p.V52A/I and p.I96V listed in genetic databases do not disturb the intracellular transport revealing that the loss of these canonical sequences may be compensated.

Conclusion

As disease-related homozygous truncating desmocollin-2 variants lacking the transmembrane domain are not localized at the plasma membrane, we predict that some of the investigated prodomain variants may be relevant in the context of arrhythmogenic cardiomyopathy due to disturbed intracellular transport.

Key Role of Astrocytes in Postnatal Brain and Retinal Angiogenesis

Angiogenesis is a key process in various physiological and pathological conditions in the nervous system and in the retina during postnatal life. Although an increasing number of studies have addressed the role of endothelial cells in this event, the astrocytes contribution in angiogenesis has received less attention. This review is focused on the role of astrocytes as a scaffold and in the stabilization of the new blood vessels, through different molecules release, which can modulate the angiogenesis process in the brain and in the retina. Further, differences in the astrocytes phenotype are addressed in glioblastoma, one of the most devastating types of brain cancer, in order to provide potential targets involved in the cross signaling between endothelial cells, astrocytes and glioma cells, that mediate tumor progression and pathological angiogenesis. Given the relevance of astrocytes in angiogenesis in physiological and pathological conditions, future studies are required to better understand the interrelation between endothelial and astrocyte signaling pathways during this process.

A genome-wide case-control association study of dark cutting in beef cattle

The genetic architecture of dark cutting was investigated with a case-control genome-wide association study on two groups of beef cattle analyzed separately and together (combined group). Groups I (n = 64) and II (n = 150) were genotyped using the 70K GeneSeek Genomic Profiler for Beef Cattle-HD and the 50K Illumina BovineSNP50v2 BeadChip, respectively. Dark cutting was analyzed as a binary trait (case versus control) using logistic regression in an additive model implemented in PLINK version 1.9. Significant loci were not identified when correcting for multiple testing (false discovery rate), suggesting that the trait is not controlled by genes with big effects, or the sample size was not large enough to detect these major genes. Regions harbouring single-nucleotide polymorphisms (SNPs) with a raw p < 0.01 using 1 MB window were analyzed for gene function using the ingenuity pathway analysis. For groups I, II, and the combined group, 449, 301, and 191 SNPs were identified, respectively. Genes identified were involved in pyruvic acid modification and release, 2-deoxyglucose clearance and disposal, sucrose recognition, energy production, and metabolism of carbohydrate. Although detected SNP associations require validation in a large population, results suggested the possibility for marker-assisted or genomic selection of beef cattle to reduce dark cutting.

The Central Role of Cadherins in Gonad Development, Reproduction, and Fertility

Cadherins are a group of membrane proteins responsible for cell adhesion. They are crucial for cell sorting and recognition during the morphogenesis, but they also play many other roles such as assuring tissue integrity and resistance to stretching, mechanotransduction, cell signaling, regulation of cell proliferation, apoptosis, survival, carcinogenesis, etc. Within the cadherin superfamily, E- and N-cadherin have been especially well studied. They are involved in many aspects of sexual development and reproduction, such as germline development and gametogenesis, gonad development and functioning, and fertilization. E-cadherin is expressed in the primordial germ cells (PGCs) and also participates in PGC migration to the developing gonads where they become enclosed by the N-cadherin-expressing somatic cells. The differential expression of cadherins is also responsible for the establishment of the testis or ovary structure. In the adult testes, N-cadherin is responsible for the integrity of the seminiferous epithelium, regulation of sperm production, and the establishment of the blood-testis barrier. Sex hormones regulate the expression and turnover of N-cadherin influencing the course of spermatogenesis. In the adult ovaries, E- and N-cadherin assure the integrity of ovarian follicles and the formation of corpora lutea. Cadherins are expressed in the mature gametes and facilitate the capacitation of sperm in the female reproductive tract and gamete contact during fertilization. The germ cells and accompanying somatic cells express a series of different cadherins; however, their role in gonads and reproduction is still unknown. In this review, we show what is known and unknown about the role of cadherins in the germline and gonad development, and we suggest topics for future research.

Expression and Prognostic Significance of Cadherin 4 (CDH4) in Renal Cell Carcinoma

Background

Aberrant expression of cadherin family members and their possible biological function have been widely studied in renal cell carcinoma (RCC). However, the expression of cadherin 4 (CDH4) and its value in RCC diagnosis and prognosis remains elusive.

Material/Methods

The TCGA database was used to analyze the expression of CDH4 and its clinical parameters and prognosis in 891 RCC patients. In addition, real-time PCR was used to verify the transcription of CDH4 in renal clear cell carcinoma tissue, and the distribution of protein was observed by immunohistochemical staining.

Results

We found that the mRNA level of CDH4 was elevated in primary RCC in contrast with normal kidney samples using bioinformatics analysis based on the TCGA database. Among the 3 main subtypes of RCC, transcriptional CDH4 was significantly increased in KIRC and KIRP, while it was downregulated in KICH. Interestingly, CDH4 mRNA gradually decreased with the progression of KIRC and KIRP. The transcription of CDH4 in the primary tumor of KIRP patients at T3–T4 stages and KIRC patients with lymph node and distant metastasis were decreased significantly. Overall survival (OS) showed that KIRC and KICH patients with lower expression of CDH4 had worse outcomes.

Conclusions

The transcriptional level of CDH4 may serve as an effective diagnostic and prognostic biomarker for RCC patients.

Cdh4 Down-Regulation Impairs in Vivo Infiltration and Malignancy in Patients Derived Glioblastoma Cells

The high invasive phenotype of glioblastoma is one of the main causes of therapy inefficacy and tumor relapse. Cell adhesion molecules of the cadherin family are involved in cell migration and are known as master regulators of epithelial tumor invasiveness, but their role in glioblastoma is less understood. In particular, we recently demonstrated, in the syngeneic murine model, the occurrence of a previously undescribed cadherin switch between Cdh2 and Cdh4 during gliomagenesis, which is necessary for the acquisition of the highly infiltrative and tumorigenic phenotype of these cells. In the present study, we tested the role of Cdh4 in human gliomas. Our results on patient-derived glioma cells demonstrate a positive correlation between Cdh4 expression levels and the loss of cell-cell contact inhibition of proliferation controls that allows cells to proliferate over confluence. Moreover, the silencing of Cdh4 by artificial microRNAs induced a decrease in the infiltrative ability of human glioma cells both in vitro and in vivo. More strikingly, Cdh4 silencing induced an impairment of the tumorigenic potential of these cells after orthotopic transplantation in immunodeficient mice. Overall, we conclude that in human glioblastoma, Cdh4 can also actively contribute in regulating cell invasiveness and malignancy.

A catenin-dependent balance between N-cadherin and E-cadherin controls neuroectodermal cell fate choices

Characterizing endogenous protein expression, interaction and function, this study identifies in vivo interactions and competitive balance between N-cadherin and E-cadherin in developing avian (Gallus gallus) neural and neural crest cells. Numerous cadherin proteins, including neural cadherin (Ncad) and epithelial cadherin (Ecad), are expressed in the developing neural plate as well as in neural crest cells as they delaminate from the newly closed neural tube. To clarify independent or coordinate function during development, we examined their expression in the cranial region. The results revealed surprising overlap and distinct localization of Ecad and Ncad in the neural tube. Using a proximity ligation assay and co-immunoprecipitation, we found that Ncad and Ecad formed heterotypic complexes in the developing neural tube, and that modulation of Ncad levels led to reciprocal gain or reduction of Ecad protein, which then alters ectodermal cell fate. Here, we demonstrate that the balance of Ecad and Ncad is dependent upon the availability of β-catenin proteins, and that alteration of either classical cadherin modifies the proportions of the neural crest and neuroectodermal cells that are specified.

Postnatal developmental dynamics of cell type specification genes in Brn3a/Pou4f1 Retinal Ganglion Cells

BACKGROUND

About 20-30 distinct Retinal Ganglion Cell (RGC) types transmit visual information from the retina to the brain. The developmental mechanisms by which RGCs are specified are still largely unknown. Brn3a is a member of the Brn3/Pou4f transcription factor family, which contains key regulators of RGC postmitotic specification. In particular, Brn3a ablation results in the loss of RGCs with small, thick and dense dendritic arbors ('midget-like' RGCs), and morphological changes in other RGC subpopulations. To identify downstream molecular mechanisms underlying Brn3a effects on RGC numbers and morphology, our group recently performed a RNA deep sequencing screen for Brn3a transcriptional targets in mouse RGCs and identified 180 candidate transcripts.

METHODS

We now focus on a subset of 28 candidate genes encoding potential cell type determinant proteins. We validate and further define their retinal expression profile at five postnatal developmental time points between birth and adult stage, using in situ hybridization (ISH), RT-PCR and fluorescent immunodetection (IIF).

RESULTS

We find that a majority of candidate genes are enriched in the ganglion cell layer during early stages of postnatal development, but dynamically change their expression profile. We also document transcript-specific expression differences for two example candidates, using RT-PCR and ISH. Brn3a dependency could be confirmed by ISH and IIF only for a fraction of our candidates.

CONCLUSIONS

Amongst our candidate Brn3a target genes, a majority demonstrated ganglion cell layer specificity, however only around two thirds showed Brn3a dependency. Some were previously implicated in RGC type specification, while others have known physiological functions in RGCs. Only three genes were found to be consistently regulated by Brn3a throughout postnatal retina development - Mapk10, Tusc5 and Cdh4.

RNA-Seq reveals seven promising candidate genes affecting the proportion of thick egg albumen in layer-type chickens

Eggs with a much higher proportion of thick albumen are preferred in the layer industry, as they are favoured by consumers. However, the genetic factors affecting the thick egg albumen trait have not been elucidated. Using RNA sequencing, we explored the magnum transcriptome in 9 Rhode Island white layers: four layers with phenotypes of extremely high ratios of thick to thin albumen (high thick albumen, HTA) and five with extremely low ratios (low thick albumen, LTA). A total of 220 genes were differentially expressed, among which 150 genes were up-regulated and 70 were down-regulated in the HTA group compared with the LTA group. Gene Ontology (GO) analysis revealed that the up-regulated genes in HTA were mainly involved in a wide range of regulatory functions. In addition, a large number of these genes were related to glycosphingolipid biosynthesis, focal adhesion, ECM-receptor interactions and cytokine-cytokine receptor interactions. Based on functional analysis, ST3GAL4, FUT4, ITGA2, SDC3, PRLR, CDH4 and GALNT9 were identified as promising candidate genes for thick albumen synthesis and metabolism during egg formation. These results provide new insights into the molecular mechanisms of egg albumen traits and may contribute to future breeding strategies that optimise the proportion of thick egg albumen.

Efficient isolation and proteomic analysis of cell plasma membrane proteins in gastric cancer reveal a novel differentiation and progression related cell surface marker, R-cadherin

Cell plasma membrane proteins, playing a crucial role in cell malignant transformation and development, were the main targets of tumor detection and therapy. In this study, CyDye/biotin double-labeling proteomic approach was adopted to profile the membrane proteome of gastric cancer cell line BGC-823 and paired immortalized gastric epithelial cell GES-1. Real-time PCR, Western blotting, and immunohistochemical staining were used to validate the differential expression of a novel identified cell surface marker R-cadherin in gastric cancer cells and tissues. Clinicopathological study and survival analysis were performed to estimate its roles in tumor progression and outcome prediction. Real-time PCR and Western blotting showed that the expression level of R-cadherin in gastric cancer were significantly lower than non-cancerous epithelial cell and tissues. Clinicopathological study indicated that R-cadherin was dominantly expressed on cell surface of normal gastric epithelium, and its expression deletion in gastric cancer tissues was associated with tumor site, differentiation, lymph node metastasis, and pTNM (chi-square test, P < 0.05). Those patients with R-cadherin positive expression displayed better overall survivals than negative expression group (log-rank test, P = 0.000). Cox multivariate survival analysis revealed lacking the expression of R-cadherin was a main independent predictor for poor clinical outcome in gastric cancer (RR = 5.680, 95 % CI 2.250–14.341, P < 0.01). We have established a fundamental membrane proteome database for gastric cancer and identified R-cadherin as a tumor differentiation and progression-related cell surface marker of gastric cancer. Lacking the expression of R-cadherin indicates poor prognosis in patients with gastric cancer.

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

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

The cellular basis of tissue separation

The subdivision of the embryo into physically distinct regions is one of the most fundamental processes in development. General hypotheses for tissue separation based on differential adhesion or tension have been proposed in the past, but with little experimental support. During the last decade, the field has experienced a strong revival, largely driven by renewed interest in biophysical modeling of development. Here, I will discuss the various models of boundary formation and summarize recent studies that have shifted our understanding of the process from the simple juxtaposition of global tissue properties to the characterization of local cellular reactions. Current evidence favors a model whereby separation is controlled by cell surface cues, which, upon cell-cell contact, generate acute changes in cytoskeletal and adhesive properties to inhibit cell mixing, and whereby the integration of multiple local cues may dictate both the global morphogenetic properties of a tissue and its separation from adjacent cell populations.

Fbxo45 inhibits calcium-sensitive proteolysis of N-cadherin and promotes neuronal differentiation

Fbxo45 is an atypical E3 ubiquitin ligase, which specifically targets proteins for ubiquitin-mediated degradation. Fbxo45 ablation results in defective neuronal differentiation and abnormal formation of neural connections; however, the mechanisms underlying these defects are poorly understood. Using an unbiased mass spectrometry-based proteomic screen, we show here that N-cadherin is a novel interactor of Fbxo45. N-cadherin specifically interacts with Fbxo45 through two consensus motifs overlapping the site of calcium-binding and dimerization of the cadherin molecule. N-cadherin interaction with Fbxo45 is significantly abrogated by calcium treatment. Surprisingly, Fbxo45 depletion by RNAi-mediated silencing results in enhanced proteolysis of N-cadherin. Conversely, ectopic expression of Fbxo45 results in decreased proteolysis of N-cadherin. Fbxo45 depletion results in dramatic reduction in N-cadherin expression, impaired neuronal differentiation, and diminished formation of neuronal processes. Our studies reveal an unanticipated role for an F-box protein that inhibits proteolysis in the regulation of a critical biological process.

Restricted expression of classic cadherins in the spinal cord of the chicken embryo

Classic cadherins belong to the family of cadherin genes and play important roles in neurogenesis, neuron migration, and axon growth. In the present study, we compared the expression patterns of 10 classic cadherins (Cdh2, Cdh4, Cdh6, Cdh7, Cdh8, Cdh9, Cdh11, Cdh12, Cdh18, and Cdh20) in the developing chicken spinal cord (SP) by in situ hybridization. Our results indicate that each of the investigated cadherins exhibits a spatially restricted and temporally regulated pattern of expression. At early developmental stages (E2.5–E3), Cdh2 is expressed throughout the neuroepithelial layer. Cdh6 is strongly positive in the roof plate and later also in the floor plate. Cdh7, Cdh11, Cdh12, and Cdh20 are expressed in restricted regions of the basal plate of the SP. At intermediate stages of development (E4–E10), specific expression profiles are observed for all investigated cadherins in the differentiating mantle layer along the dorsoventral, mediolateral, and rostrocaudal dimensions. Expression profiles are especially diverse for Cdh2, Cdh4, Cdh8, Cdh11, and Cdh20 in the dorsal horn, while different pools of motor neurons exhibit signal for Cdh6, Cdh7, Cdh8, Cdh9, Cdh12, and Cdh20 in the ventral horn. Interestingly, subpopulations of cells in the dorsal root ganglion express combinations of different cadherins. In the surrounding tissues, such as the boundary cap cells and the notochord, the cadherins are also expressed differentially. The highly regulated spatiotemporal expression patterns of the classic cadherins indicate that these genes potentially play multiple and diverse roles during the development of the SP and its surrounding tissues.

ADAM10 mediates N-cadherin ectodomain shedding during retinal ganglion cell differentiation in primary cultured retinal cells from the developing chick retina

Here, we examined the role of ADAM10 during retinal cell differentiation in retinal sections and in vitro cultures of developing chick retinal cells from embryonic day 6 (ED6). Immunohistochemistry showed that ADAM10 is abundantly expressed in the inner zone of neuroblastic layer at ED5, and it becomes more highly expressed in the ganglion cell layer at ED7 and ED9. Western blotting confirmed that ADAM10 was expressed as an inactive pro-form that was processed to a shorter, active form in control cultured cells, but in cultures treated with an ADAM10 inhibitor (GI254023X) and ADAM10-specific siRNA, the level of mature ADAM10 decreased. Phase-contrast microscopy showed that long neurite extensions were present in untreated cultures 24 h after plating, whereas cultures treated with GI254023X showed significant decreases in neurite extension. Immunofluorescence staining revealed that there were far fewer differentiated ganglion cells in ADAM10 siRNA and GI254023X-treated cultures compared to controls, whereas the photoreceptor cells were unaltered. The Pax6 protein was more strongly detected in the differentiated ganglion cells of control cultures compared to ADAM10 siRNA and GI254023X-treated cultures. N-cadherin ectodomain shedding was apparent in control cultures after 24 h, when ganglion cell differentiation was observed, but ADAM10 siRNA and GI254023X treatment inhibited these processes. In contrast, N-cadherin staining was strongly detected in photoreceptor cells regardless of ADAM10 siRNA and GI254023X treatment. Taken together, these data indicate that the inhibition of ADAM10 can inhibit Pax6 expression and N-cadherin ectodomain shedding in retinal cells, possibly affecting neurite outgrowth and ganglion cell differentiation.

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.

Cadherins in brain morphogenesis and wiring

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

Differential expression of the ADAMs in developing chicken retina

The expression patterns of the seven members of the ADAM (a disintegrin and metalloprotease) family, ADAM9, ADAM10, ADAM12, ADAM13, ADAM17, ADAM22, and ADAM23 were analyzed in the developing chicken retina by in situ hybridization and immunohistochemistry. Results show that each individual ADAM is expressed and regulated spatiotemporally in the developing retinal layers. ADAM9, ADAM10 and ADAM17 are widely expressed in the differential layers of the retina throughout the whole embryonic period, while ADAM12 and ADAM13 are mainly expressed in the ganglion cell layer at a later stage. ADAM22 and ADAM23 are restricted to the inner nuclear layer and the ganglion cell layer at a later stage. Furthermore, ADAM10 protein is co-expressed with the four members of the classic cadherins, N-cadherin, R-cadherin, cadherin-6B and cadherin-7 in distinct retinal layers. Therefore, the differential expression of the investigated ADAMs in the developing retina suggests the contribution of them to the retina development.

Molecular characterization and expression analysis of ADAM12 during chicken embryonic development

ADAM12 is a member of the disintegrin and metalloprotease (ADAM) family of molecules, which consist of multiple domains. ADAM12 is involved in different physiological and pathological processes. In the present study, full-length sequences of two chicken ADAM12 isoforms were cloned and identified by reverse transcription-polymerase chain reaction (RT-PCR), rapid amplification of cDNA ends methods and bioinformatics analysis. The long isoform consists of all domains characteristic for ADAMs and is strongly expressed in different tissues, whereas the short isoform lacks large parts of the metalloprotease and disintegrin domains and is only expressed weakly. Results from semi-quantitative RT-PCR show that the complete ADAM12 is stably expressed throughout chicken embryonic development, while the short isoform is only regionally detectable in the lung and brain. Results from in situ hybridization show that chicken ADAM12 is expressed exclusively in tissues and organs derived from the neural tube, the neural crest or the mesoderm, with a highly regulated spatiotemporal expression pattern. Our data confirm and extend studies of ADAM12 in other species, and suggest that ADAM12 may play a role in the development of several organs, including the formation of feather buds.

Comparative gene expression analysis among vocal learners (bengalese finch and budgerigar) and non-learners (quail and ring dove) reveals variable cadherin expressions in the vocal system

Birds use various vocalizations to communicate with one another, and some are acquired through learning. So far, three families of birds (songbirds, parrots, and hummingbirds) have been identified as having vocal learning ability. Previously, we found that cadherins, a large family of cell-adhesion molecules, show vocal control-area-related expression in a songbird, the Bengalese finch. To investigate the molecular basis of evolution in avian species, we conducted comparative analysis of cadherin expressions in the vocal and other neural systems among vocal learners (Bengalese finch and budgerigar) and a non-learner (quail and ring dove). The gene expression analysis revealed that cadherin expressions were more variable in vocal and auditory areas compared to vocally unrelated areas such as the visual areas among these species. Thus, it appears that such diverse cadherin expressions might have been related to generating species diversity in vocal behavior during the evolution of avian vocal learning.

p120-Catenin is essential for N-cadherin-mediated formation of proper junctional structure, thereby establishing cell polarity in epithelial cells

The role of p120-catenin in the function of classical cadherins is still enigmatic despite various studies. To elucidate its role, we examined the effect of p120-catenin on the N-cadherin-mediated localization of junctional proteins in epithelial cells in this study. Cadherin-deficient MIA PaCa-2 epithelial cells did not show linear localization of tight junction proteins ZO-1 and occludin. When N-cadherin was expressed in these cells, however, the resultant transfectant cells revealed strong cell adhesion activity and linear localization of ZO-1, occludin, and N-cadherin in the lateral membrane. When the p120-catenin-binding site of N-cadherin was disrupted, the linear localization of ZO-1 and occludin disappeared, and the mutant N-cadherin became localized more diffusely in the transfectant, although the cell adhesion activity did not change much. Knockdown of p120-catenin also resulted in the very weak localization of ZO-1 and occludin. A similar effect of p120-catenin on the localization of junctional proteins was obtained under more dynamic conditions in a wound healing assay. Moreover, p120-catenin was essential for the regulation of centrosome orientation in this healing assay. Taken together, the present data indicate that p120-catenin is essential for N-cadherin-mediated formation of proper junctional structures and thereby the establishment of the cell polarity. Similar results were obtained when E-cadherin mutants comparable to those of N-cadherin were used, suggesting that p120-catenin plays the same role in the function of other classical cadherins.

Adhesion properties and retinofugal expression of chicken protocadherin-19

Protocadherin-19 has been implicated in some neurological diseases, but even the basic properties of this protocadherin have not yet been characterized well. Hence, various basic properties of chicken protocadherin-19 were examined to elucidate its biological role. The protocadherin-19 expressed in L cells was localized at the intercellular contact sites and showed Ca(2+)-dependent homophilic cell aggregation activity that was relatively weak but showed stringent specificity. The results of a pull-down assay using fusion proteins of the cytoplasmic domain and glutathione S-transferase yielded specifically bound proteins. In the bound fractions, liquid chromatography-mass spectrometry identified Nck-associated protein 1 and cytoplasmic FMP1 interacting protein 2, which have been reported to bind to glutathione S-transferase fused with the cytoplasmic domain of OL-protocadherin, suggesting that these proteins generally have affinity for delta protocadherins. Protocadherin-19 was mainly expressed in the central nervous system. In the chicken retina, protocadherin-19 was expressed as early as embryonic day 5 and was localized in the ganglion cell layer, inner plexiform layer, and optic nerve layer. Chicken protocadherin-19 was co-localized with syntaxin 1 in inner plexiform layer and was also expressed in the optic nerve and in specific layers of optic tectum. These results suggest that protocadherin-19 plays a role as an adhesion protein in optic nerve fiber bundling, optic nerve targeting, and/or synapse formation.

Distinct PTPmu-associated signaling molecules differentially regulate neurite outgrowth on E-, N-, and R-cadherin

Classical cadherins play distinct roles in axon growth and guidance in the visual system, however, the signaling pathways they activate remain unclear. Growth cones on each cadherin substrate have a unique morphology suggesting that distinct signals are activated by neurite outgrowth on E-, N-, and R-cadherin. We previously demonstrated that receptor protein tyrosine phosphatase-mu (PTPmu) is required for E- and N-cadherin-dependent neurite outgrowth. In this manuscript, we demonstrate that PTPmu regulates R-cadherin-mediated neurite outgrowth. Furthermore, we evaluated whether known PTPmu-associated signaling proteins, Rac1, Cdc42, IQGAP1 and PKCdelta, regulate neurite outgrowth mediated by these cadherins. While Rac1 activity is required for neurite outgrowth on all three cadherins Cdc42/IQGAP1 are required only for N- and R-cadherin-mediated neurite outgrowth. In addition, we determined that PKC activity is required for E- and R-cadherin-mediated, but not N-cadherin-mediated neurite outgrowth. In summary, distinct PTPmicro-associated signaling proteins are required to promote neurite outgrowth on cadherins.

Expression of classic cadherins and delta-protocadherins in the developing ferret retina

BACKGROUND

Cadherins are a superfamily of calcium-dependent adhesion molecules that play multiple roles in morphogenesis, including proliferation, migration, differentiation and cell-cell recognition. The subgroups of classic cadherins and delta-protocadherins are involved in processes of neural development, such as neurite outgrowth, pathfinding, target recognition, synaptogenesis as well as synaptic plasticity. We mapped the expression of 7 classic cadherins (CDH4, CDH6, CDH7, CDH8, CDH11, CDH14, CDH20) and 8 delta-protocadherins (PCDH1, PCDH7, PCDH8, PCDH9, PCDH10, PCDH11, PCDH17, PCDH18) at representative stages of retinal development and in the mature retina of the ferret by in situ hybridization.

RESULTS

All cadherins investigated by us are expressed differentially by restricted populations of retinal cells during specific periods of the ferret retinogenesis. For example, during embryonic development, some cadherins are exclusively expressed in the outer, proliferative zone of the neuroblast layer, whereas other cadherins mark the prospective ganglion cell layer or cells in the prospective inner nuclear layer. These expression patterns anticipate histogenetic changes that become visible in Nissl or nuclear stainings at later stages. In parallel to the ongoing development of retinal circuits, cadherin expression becomes restricted to specific subpopulations of retinal cell types, especially of ganglion cells, which express most of the investigated cadherins until adulthood. A comparison to previous results in chicken and mouse reveals overall conserved expression patterns of some cadherins but also species differences.

CONCLUSIONS

The spatiotemporally restricted expression patterns of 7 classic cadherins and 8 delta-protocadherins indicate that cadherins provide a combinatorial adhesive code that specifies developing retinal cell populations and intraretinal as well as retinofugal neural circuits in the developing ferret retina.

Biophysical properties of cadherin bonds do not predict cell sorting

Differential binding between cadherin subtypes is widely believed to mediate cell sorting during embryogenesis. However, a fundamental unanswered question is whether cell sorting is dictated by the biophysical properties of cadherin bonds, or by broader, cadherin-dependent differences in intercellular adhesion or membrane tension. This report describes atomic force microscope measurements of the strengths and dissociation rates of homophilic and heterophilic cadherin (CAD) bonds. Measurements conducted with chicken N-CAD, canine E-CAD, and Xenopus C-CAD demonstrated that all three cadherins cross-react and form multiple, intermolecular bonds. The mechanical and kinetic properties of the heterophilic bonds are similar to the homophilic interactions. The thus quantified bond parameters, together with previously reported adhesion energies were further compared with in vitro cell aggregation and sorting assays, which are thought to mimic in vivo cell sorting. Trends in quantified biophysical properties of the different cadherin bonds do not correlate with sorting outcomes. These results suggest that cell sorting in vivo and in vitro is not governed solely by biophysical differences between cadherin subtypes.

R-Cadherin expression inhibits myogenesis and induces myoblast transformation via Rac1 GTPase

Cadherins are transmembrane glycoproteins that mediate Ca(2+)-dependent homophilic cell-cell adhesion and play a crucial role in proliferation, differentiation, and cell transformation. The goal of this study was to understand why R-cadherin is found in rhabdomyosarcomas (RMS), tumors of skeletal muscle origin, whereas it is absent in normal myoblasts. We show that R-cadherin expression in C2C12 myoblasts causes inhibition of myogenesis induction and impairment of cell cycle exit when cells are cultured in differentiation medium. Furthermore, R-cadherin expression elicits myoblast transformation, as shown by anchorage-independent growth in soft agar in vivo tumor formation assays and increased cell motility. In contrast, inhibition of R-cadherin expression using RNA interference hinders growth of RD cell line in soft agar and its tumorigenicity in mice. The analysis of the nature of R-cadherin-mediated signals shows that R-cadherin-dependent adhesion increases Rac1 activity. Dominant-negative forms of Rac1 inhibit R-cadherin-mediated signaling and transformation. In addition, expression of R-cadherin results in perturbed function of endogenous N-cadherin and M-cadherin. Together, these data suggest that R-cadherin expression inhibits myogenesis and induces myoblast transformation through Rac1 activation. Therefore, the properties of R-cadherin make it an attractive target for therapeutic intervention in RMS.

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.

Dscam and Sidekick proteins direct lamina-specific synaptic connections in vertebrate retina

Synaptic circuits in the retina transform visual input gathered by photoreceptors into messages that retinal ganglion cells (RGCs) send to the brain. Processes of retinal interneurons (amacrine and bipolar cells) form synapses on dendrites of RGCs in the inner plexiform layer (IPL). The IPL is divided into at least 10 parallel sublaminae; subsets of interneurons and RGCs arborize and form synapses in just one or a few of them. These lamina-specific circuits determine the visual features to which RGC subtypes respond. Here we show that four closely related immunoglobulin superfamily (IgSF) adhesion molecules--Dscam (Down's syndrome cell adhesion molecule), DscamL (refs 6-9), Sidekick-1 and Sidekick-2 (ref. 10)--are expressed in chick by non-overlapping subsets of interneurons and RGCs that form synapses in distinct IPL sublaminae. Moreover, each protein is concentrated within the appropriate sublaminae and each mediates homophilic adhesion. Loss- and gain-of-function studies in vivo indicate that these IgSF members participate in determining the IPL sublaminae in which synaptic partners arborize and connect. Thus, vertebrate Dscams, like Drosophila Dscams, play roles in neural connectivity. Together, our results on Dscams and Sidekicks suggest the existence of an IgSF code for laminar specificity in retina and, by implication, in other parts of the central nervous system.

Structure and Mechanism of Cadherins and Catenins in Cell-Cell Contacts

Cadherins are Ca(2+)-dependent cell adhesion molecules found in several kinds of cell-cell contact, including adherens junctions and desmosomes. In the presence of Ca(2+), cells expressing the same type of cadherin form stable contacts with one another, a phenomenon designated homophilic, or homotypic, adhesion. Most cadherins are single-pass transmembrane proteins whose extracellular regions mediate specific cell-cell interactions. The intracellular faces of these contacts are associated with the actin cytoskeleton in adherens junctions or the intermediate-filament system in desmosomes. The close coordination of the transmembrane adhesion molecules with the cytoskeleton is believed to be essential in coordinating morphogenetic movements of tissues during development and in conferring the appropriate mechanical properties to cell-cell contacts. Structural, biochemical, and biophysical analysis of the molecules that comprise these contacts has provided unique mechanistic insights into the specificity of homophilic adhesion, the functional connection to the underlying cytoskeleton, and the dynamics of junction formation.

Cadherin-4 plays a role in the development of zebrafish cranial ganglia and lateral line system

We previously reported that cadherin-4 (also called R-cadherin) was expressed by the majority of the developing zebrafish cranial and lateral line ganglia. Cadherin-4 (Cdh4) function in the formation of these structures in zebrafish was studied using morpholino antisense technology. Differentiation of the cranial and lateral line ganglia and lateral line nerve and neuromasts of the cdh4 morphants was analyzed using multiple neural markers. We found that a subset of the morphant cranial and lateral line ganglia were disorganized, smaller, with reduced staining, and/or with altered shape compared to control embryos. Increased cell death in the morphant ganglia likely contributed to these defects. Moreover, cdh4 morphants had shorter lateral line nerves and a reduced number of neuromasts, which was likely caused by disrupted migration of the lateral line primordia. These results indicate that Cdh4 plays a role in the normal formation of the zebrafish lateral line system and a subset of the cranial ganglia.

L1, beta1 integrin, and cadherins mediate axonal regeneration in the embryonic spinal cord

Embryonic birds and mammals are capable of axon regeneration after spinal cord injury, but this ability is lost during a discrete developmental transition. We recently showed that changes within maturing neurons, as opposed to changes solely in the spinal cord environment, significantly restrict axon regeneration during development. The developmental changes within neurons that limit axon regeneration remain unclear. One gap in knowledge is the identity of the adhesive receptors that embryonic neurons use to extend axons in the spinal cord. Here we test the roles of L1/NgCAM, beta1 integrin, and cadherins, using a coculture system in which embryonic chick brainstem neurons regenerate axons into an explant of embryonic spinal cord. By in vivo and in vitro methods, we found that brainstem neurons reduce axonal expression of L1 as they mature. Disrupting either L1 or beta1 integrin function individually in our coculture system partially inhibited growth of brainstem axons in spinal cords, while disrupting cadherin function alone had no effect. However, when all three adhesive receptors were blocked simultaneously, axon growth in the spinal cord was reduced by 90%. Using immunohistochemistry and in situ hybridization we show that during the period when neurons lose their regenerative capacity they reduce expression of mRNA for N-cadherin, and reduce axonal L1/NgCAM protein through a post-transcriptional mechanism. These data show that embryonic neurons use L1/NgCAM, beta1 integrin, and cadherin receptors for axon regeneration in the embryonic spinal cord, and raise the possibility that a reduced expression of these essential receptors may contribute to the low-regenerative capacity of older neurons.

The CNS synapse revisited: gaps, adhesive welds, and borders

Although processes leading up to the point of synapse formation are fairly well understood, the precise sequence of events in which the membranes of two separate cells "lock in" to form a mature synaptic junctional complex is poorly understood. A careful study of the molecules operating at the synapse indicates that their roles are more multifarious than once imagined. In this review we posit that the synapse is a functional organelle with poorly defined boundaries and a complex biochemistry. The role of adhesion molecules, including the integration of their signaling and adhesive properties in the context of synaptic activity is discussed.

Antiparasitic effect of calcium and magnesium ion-free buffer treatments against a common monogenean Neobenedenia girellae

This study investigated a new effective method for controlling the capsalid monogenean Neobenedenia girellae. We examined in vitro and in vivo the effect on the percentage survival of N. girellae in buffers containing different metallic ions. Decreased survival was observed in buffer solutions lacking two ions. In particular, the percentage survival of N. girellae was significantly decreased after 10 min exposure to buffer containing neither Ca(2+) nor Mg(2+). Transmission electron microscopic observations showed that treatment with this buffer disrupted intercellular junctions. This significant effect on percentage survival of N. girellae using Ca(2+)/Mg(2+)-free buffer was confirmed in an in vivo assay. Ca(2+)/Mg(2+)-free buffer had no effect on the condition of the host, spotted halibut Verasper variegates (Pleuronectidae). These results suggest that treatment with Ca(2+)/Mg(2+)-free buffer is a new effective control method, which could replace existing control methods.

Mechanism and dynamics of cadherin adhesion

Cadherins are essential cell adhesion molecules involved in tissue morphogenesis and the maintenance of tissue architecture in adults. The adhesion and selectivity functions of cadherins are located in their extracellular regions. Biophysical studies show that the adhesive activity is not confined to a single interface. Instead, multiple cadherin domains contribute to binding. By contrast, the specificity-determining site maps to the N-terminal domains, which adhere by the reciprocal binding of Trp2 residues from opposing proteins. Structural cooperativity can transmit the effects of subtle structural changes or ligand binding over large distances in the protein. Increasingly, studies show that differential cadherin-mediated adhesion, rather than exclusive homophilic binding between identical cadherins, direct cell segregation and the organization of tissue interfaces during morphogenesis. Force measurements quantified both kinetic and strength differences between different classical cadherins that may underlie cell sorting behavior. Despite the complex adhesion mechanisms and differences in binding properties, cadherin-mediated cell adhesion is also regulated by many other biochemical processes. Elucidating the mechanisms by which cadherins organize cell junctions and tissue architecture requires not only quantitative, mechanistic investigations of cadherin function but also investigations of the biochemical and cellular processes that can modulate those functions.

Zebrafish R-cadherin (Cdh4) controls visual system development and differentiation

In zebrafish, R-cadherin (cadherin-4 or Cdh4) is expressed in the retina and in retinorecipient brain regions, suggesting that Cdh4 functions during visual system development. Cdh4 function was examined during retinogenesis and retinal axon outgrowth using antisense morpholino oligonucleotides and mutant Cdh4 construct expression. In knockdowns, Cdh4 was reduced or absent, eyes were small, and retinae lacked discrete laminae. Increased cell death produced the small eye phenotype. Zn5-, Pax6-, and zpr-1-positive cells were reduced or absent in knockdown retinas but, when present, were in the correct laminae. Cdh4 knockdowns had sparse or absent retinal ganglion cell axons. When present, axons projected contralaterally but lacked fine branching and failed to reach the tectum or arborize the entire tectum. Mutant Cdh4 construct expression during retinal ganglion cell differentiation reduced or ablated neurite formation. Cdh4 is necessary for neural retina survival and differentiation, and required for normal retinotectal projection formation and tectal arborization.

Localization and expression of R-cadherin in skeletal muscle following nerve injury

Rats in which the sciatic nerves were cut were divided into two groups: animals with nerve sutured and animals with nerve not sutured. In the unsutured group, the levels of R-cadherin expression increased and then decreased to values lower than those of controls. In the sutured group, the levels of R-cadherin expression increased and then decreased to almost control values. These results suggest that R-cadherin plays some role in cells of normal and regenerating muscles.

The differential adhesion hypothesis: a direct evaluation

The differential adhesion hypothesis (DAH), advanced in the 1960s, proposed that the liquid-like tissue-spreading and cell segregation phenomena of development arise from tissue surface tensions that in turn arise from differences in intercellular adhesiveness. Our earlier measurements of liquid-like cell aggregate surface tensions have shown that, without exception, a cell aggregate of lower surface tension tends to envelop one of higher surface tension to which it adheres. We here measure the surface tensions of L cell aggregates transfected to express N-, P- or E-cadherin in varied, measured amounts. We report that in these aggregates, in which cadherins are essentially the only cell-cell adhesion molecules, the aggregate surface tensions are a direct, linear function of cadherin expression level. Taken together with our earlier results, the conclusion follows that the liquid-like morphogenetic cell and tissue rearrangements of cell sorting, tissue spreading and segregation represent self-assembly processes guided by the diminution of adhesive-free energy as cells tend to maximize their mutual binding. This conclusion relates to the physics governing these morphogenetic phenomena and applies independently of issues such as the specificities of intercellular adhesives.

Novel soluble molecule, Akhirin, is expressed in the embryonic chick eyes and exhibits heterophilic cell-adhesion activity

Akhirin, a novel secreted protein of 90 kDa, has been identified using signal sequence trap cDNA screening of an embryonic day 6 chicken lens cDNA library. Akhirin consists of one LCCL (Limulus factor C, Coch-5b2, and Lgl1) domain and two von Willebrand factor domains and displays high structural homology to vitrin and cochlin. The earliest expression of Akhirin is observed in the head ectoderm overlying the lens vesicle at stage 17 and in the retinal pigment epithelial layer at stage 22. It is persistently expressed in the ciliary marginal zone and in lens epithelium cells throughout embryonic eye development. Immunostaining with anti-Akhirin monoclonal antibody revealed a punctate distribution of Akhirin protein on living transfected cells. Cell adhesion and cell aggregation experiments showed that Akhirin has heterophilic cell-adhesion activity. Based on these observations, we hypothesize that Akhirin is involved in eye development.

R-Cadherin:β-Catenin Complex and Its Association With Vascular Smooth Muscle Cell Proliferation

OBJECTIVE

Vascular smooth muscle cell (VSMC) proliferation is an important component of atherosclerosis, restenosis after angioplasty and stent placement, and vein graft failure. Outside-in signaling from the cadherin:beta-catenin complex can increase transcription of the cell-cycle gene cyclin D1; however, its role in VSMC proliferation has only recently been considered.

METHODS AND RESULTS

We examined the involvement of R-cadherin and beta-catenin in VSMC proliferation in balloon-injured carotid arteries in vivo and aortic rings in vitro. The number of medial VSMCs positive for R-cadherin was significantly reduced by 32%+/-5%, 52%+/-10%, and 23%+/-2% at 0.25, 24, and 48 hours after injury in vivo, respectively. These changes in cadherin expression coincided with the detection of nuclear beta-catenin and elevated cyclin D1 expression. Furthermore, loss of R-cadherin expression was associated with medial VSMC proliferation. Inhibition of classical cadherin function with a HAV peptide and R-cadherin neutralizing antibodies significantly increased proliferation by 4.3+/-1.0-fold and 4.1+/-0.98-fold, and increased the number of cells with beta-catenin in the nucleus and expressing cyclin D1 in aortic rings.

CONCLUSIONS

These results suggest that R-cadherin expression and beta-catenin signaling may be associated with increased cyclin D1 expression and VSMC proliferation and may therefore play an important role in vascular disease.

Adult bone marrow-derived stem cells use R-cadherin to target sites of neovascularization in the developing retina

Adult bone marrow contains a population of hematopoietic stem cells (HSCs) that can give rise to cells capable of targeting sites of neovascularization in the peripheral or retinal vasculature. However, relatively little is known about the mechanism of targeting of these cells to sites of neovascularization. We have analyzed subpopulations of HSCs for the expression of a variety of cell surface adhesion molecules and found that R-cadherin, a calcium-dependent cell-cell adhesion molecule important for normal retinal endothelial cell guidance, was preferentially expressed by functionally targeting HSCs. Preincubation of HSCs with function-blocking anti-R-cadherin antibodies or novel R-cadherin-specific peptide antagonists effectively prevented targeting of bone marrow-derived cells to the developing retinal vasculature in vivo. Whereas control-injected HSCs targeted to all 3 normal developing retinal vascular layers, blocking R-cadherin-mediated adhesion resulted in mistargeting of the HSCs to the normally avascular outer retina. Our results suggest that vascular targeting of bone marrow-derived HSCs is dependent on mechanisms similar to those used by endogenous retinal vascular endothelial cells. Thus, R-cadherin antagonists may be useful in the treatment of neovascular diseases in which circulating HSCs contribute to abnormal angiogenesis. (Blood. 2004;103:3420-3427)

OL-protocadherin expression in the visual system of the chicken embryo

The expression of OL-protocadherin, a homotypically binding cell adhesion molecule, was mapped in the visual system of the chicken embryo at intermediate to late stages of development (11-19 days of incubation). The expression was compared with that of four classic cadherins, described previously. OL-protocadherin is expressed by the isthmooptic nucleus, its retinopetal projection, and possibly its retinal target neurons, the amacrine cells. Ganglion cells begin to express OL-protocadherin at relatively late stages of development. The layers of the optic tectum, the projection neurons in the stratum griseum centrale, and the tectofugal pathways show differential OL-protocadherin immunoreactivity. Several of the diencephalic target nuclei of the tectothalamic projection, such as the principal pretectal nucleus, subpretectal nucleus, and nucleus rotundus, contain distinct subregions or populations of neurons expressing OL-protocadherin. In these centers, the expression pattern of OL-protocadherin differs from that of the four classic cadherins, though it shows partial overlap with them. Other retinorecipient and/or tectorecipient nuclei (ventral geniculate nucleus, lateral dorsolateral nucleus, superficial synencephalic nucleus, pretectal area, and griseum tectale) also show a differential immunoreactivity for OL-protocadherin and other cadherins. Some of these nuclei and the optic tectum display a similar sequence of cadherin expression from superficial to deep layers, in a pattern that may reflect mutual interconnections. This result indicates a partial conservation of cadherin expression across interconnected embryonic divisions, from the mesencephalon to the ventral thalamus. In conclusion, OL-protocadherin is a marker for specific functional gray matter structures and neural circuits in the chicken visual system. J. Comp. Neurol. 470:240-255, 2004.

Selective synaptic cadherin expression by traced neurons of the chicken visual system

The stable and specific locking-in of pre- and postsynaptic membranes in synaptogenesis may be mediated by integral membrane proteins, such as members of the cadherin family. Cadherins are ideal candidate molecules for mediating synaptic specificity because they are differentially expressed in functionally connected brain structures. We studied the expression of four classic cadherins (R-cadherin, N-cadherin, cadherin-6B and cadherin-7) at the synaptic level on the somata and the proximal neurites of identified neuron populations that were traced selectively in the developing chicken visual system. Three major findings were observed. (1) Synapses on somata of shepherd's crook cells of the optic tectum are associated preferentially with one cadherin subtype. (2) In an isthmic nucleus that contains a mixed population of cells expressing different cadherins, somatic synapses tend to express the same cadherin subtype as the rest of the cell. (3) In the oculomotor complex, two cadherin subtypes are expressed only by synapses on the axon hillock. However, another neuron type that projects from the tectum to the isthmic nucleus does not show such selective synaptic cadherin staining. Our findings support the idea that a cadherin-based adhesive mechanism can mediate synaptic specificity.

Internal structure of the nucleus rotundus revealed by mapping cadherin expression in the embryonic chicken visual system

The nucleus rotundus is the largest nucleus of the avian thalamus. It is an important center of visual information processing and conveys information from the optic tectum to the ectostriatum in the telencephalon. The nucleus rotundus is generally believed to contain internal divisions processing information on color, form, motion, and looming of visual objects. The detailed arrangement of these internal divisions is unclear. Here, we map the expression of four classic cadherins (N-cadherin, R-cadherin, cadherin-6B, and cadherin-7), which are markers for specific functional gray matter divisions and their fiber connections in the vertebrate brain. Results show that each cadherin is expressed by one coherent part of the nucleus rotundus that is connected to other brain structures by fiber tracts expressing the same subtype of cadherin. Overall, the expression of the four cadherins encompasses almost the entire nucleus rotundus. The four cadherin-expressing parts show different degrees of overlap. For example, the cadherin-6B part and the cadherin-7 part overlap extensively, whereas the R-cadherin part and the cadherin-6B part show little overlap and are partially complementary. Regions with shallow gradients of cadherin expression alternate with regions that show relatively abrupt changes in cadherin expression. At some points, changes of cadherin expression are also arranged in a pinwheel-like fashion, alternating between clockwise and counterclockwise orientations. In general, these results are reminiscent of the organization of functional modules in the mammalian visual cortex. It is speculated that each domain of cadherin expression corresponds to a functional domain, which processes a specific stimulus feature.

Ebf gene function is required for coupling neuronal differentiation and cell cycle exit

Helix-loop-helix transcription factors of the Ebf/Olf1 family have previously been implicated in the control of neurogenesis in the central nervous system in both Xenopus laevis and the mouse, but their precise roles have remained unclear. We have characterised two family members in the chick, and have performed a functional analysis by gain- and loss-of-function experiments. This study revealed several specific roles for Ebf genes in the spinal cord and hindbrain regions of higher vertebrates, and enabled their precise positioning along the neurogenic cascade. During neurogenesis, cell cycle exit appears to be tightly coupled to migration to the mantle layer and to neuronal differentiation. We show that antagonizing Ebf gene activity allows the uncoupling of these processes. Ebf gene function is necessary to initiate neuronal differentiation and migration toward the mantle layer in neuroepithelial progenitors, but it is not required for cell cycle exit. Ebf genes therefore appear to be master controllers of neuronal differentiation and migration, coupling them to cell cycle exit and earlier steps of neurogenesis. Mutual activation between proneural and Ebf genes suggests that besides their involvement in the engagement of differentiation, Ebf genes may also participate in the stabilisation of the committed state. Finally, gain-of-function data raise the possibility that, in addition to these general roles, Ebf genes may be involved in neuronal subtype specification in particular regions of the CNS.

Cadherin-mediated cell adhesion and tissue segregation: qualitative and quantitative determinants

It is widely held that segregation of tissues expressing different cadherins results from cadherin-subtype-specific binding specificities. This belief is based largely upon assays in which cells expressing different cadherin subtypes aggregate separately when shaken in suspension. In various combinations of L cells expressing NCAM, E-, P-, N-, R-, or B-cadherin, coaggregation occurred when shear forces were low or absent but could be selectively inhibited by high shear forces. Cells expressing P- vs E-cadherin coaggregated and then demixed, one population enveloping the other completely. To distinguish whether this demixing was due to differences in cadherin affinities or expression levels, the latter were varied systematically. Cells expressing either cadherin at a lower level became the enveloping layer, as predicted by the Differential Adhesion Hypothesis. However, when cadherin expression levels were equalized, cells expressing P- vs E-cadherin remained intermixed. In this combination, "homocadherin" (E-E; P-P) and "heterocadherin" (E-P) adhesions must therefore be of similar strength. Cells expressing R- vs B-cadherin coaggregated but demixed to produce configurations of incomplete envelopment. This signifies that R- to B-cadherin adhesions must be weaker than either "homocadherin" adhesion. Together, cadherin quantity and affinity control tissue segregation and assembly through specification of the relative intensities of mature cell-cell adhesions.