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.

A combination of enhancer/silencer modules regulates spatially restricted expression of cadherin-7 in neural epithelium.

The spatially restricted expression of cadherin-7 to the intermediate domain of the neural epithelium and in migrating neural crest cells during early neural development is potentially regulated by multiple signaling inputs. To identify the regulatory modules involved in regulation of cadherin-7, evolutionary conserved non-coding sequences in the cadherin-7 locus were analyzed. This led to the identification of an evolutionary conserved region of 606 bp (ECR1) that together with the cadherin-7 promoter recapitulates endogenous cadherin-7 expression in intermediate neural tube, spinal motor neurons, interneurons, and dorsal root ganglia. Deletion analysis of ECR1 revealed a 19-bp block that is essential for ECR1 enhancer activity, while two separate blocks of 10 and 12 bp were found to be essential for ECR1 silencer activity in the dorsal and ventral neural epithelium, respectively. Together, these data provide an insight into tissue-specific regulatory regions that might be involved in regulation of cadherin-7 gene expression.

Regulation of cadherin expression in the chicken neural crest by the Wnt/ß-catenin signaling pathway.

In neural crest cell development, the expression of the cell adhesion proteins cadherin-7 and cadherin-11 commences after delamination of the neural crest cells from the neuroepithelium. The canonical Wnt signaling pathway is known to drive this delamination step and is a candidate for inducing expression of these cadherins at this time. This project was initiated to investigate the role of canonical Wnt signaling in the expression of cadherin-7 and cadherin-11 by treating neural crest cells with Wnt3a ligand. Expression of cadherin-11 was first confirmed in the neural crest cells for the chicken embryo. The changes in the expression level of cadherin-7 and -11 following the treatment with Wnt3a were studied using real-time RT-PCR and immunostaining. Statistically significant upregulation in the mRNA expression of cadherin-7 and cadherin-11 and in the amount of cadherin-7 and cadherin-11 protein found in cell-cell interfaces between neural crest cells was observed in response to Wnt, demonstrating that cadherin-7 and cadherin-11 expressed by the migrating neural crest cells can be regulated by the canonical Wnt pathway.

Myotonic dystrophy protein kinase is expressed in embryonic myocytes and is required for myotube formation.

Myotonic dystrophy (DM1) is a multi-systemic disease caused by a triplet nucleotide repeat expansion in the 3' untranslated region of the gene coding for myotonic dystrophy protein kinase (DMPK). The primary pathophysiology of DM1 is thought to result from RNA transport and processing defects. The function of DMPK in development or any potential role in DM1 remains unknown. Here we report a novel role for DMPK in myogenesis. We have discovered a specific expression pattern of DMPK in mouse and chick embryonic development. DMPK is expressed in postmitotic cardiac and skeletal myocytes and developmental signaling centers. During cardiac myocyte maturation, DMPK migrates from perinuclear to cellular membrane localization. Manipulating DMPK levels in cultured cardiac and skeletal myocytes has revealed a key role for DMPK in myocyte differentiation. Overexpression of DMPK induces cell rounding and apoptosis in myocytes. In addition, DMPK is necessary for myogenin expression in differentiating C2C12 myoblasts.

Gap junctions assemble in the presence of cytoskeletal inhibitors, but enhanced assembly requires microtubules.

The role of cytoskeletal elements in gap junction (GJ) assembly has been studied using Novikoff hepatoma cells treated with cytochalasin B (CB) to disrupt actin filaments or with colchicine or nocodazole to disrupt microtubules. After 60 min of cell reaggregation, freeze-fracture was used to evaluate quantitatively the "initiation," "maturation," and "growth" phases of GJ assembly. The development of junctional permeability to fluorescent dyes was also analyzed. The only effects of CB on the structure or permeability of the developing junctions involved an elongation of GJ aggregates and a small decrease in formation plaque areas. Colchicine (but not the inactive form, lumicolchicine) prevented the enhancement of GJ growth by cholesterol, but its effect on basal growth was equivocal. Nocodazole inhibited the growth of GJ, even under basal conditions, without an effect on initiation. Nocodazole also blocked the forskolin-enhanced increase in the growth of GJs and, in living MDCK cells, reduced the movement of transport intermediates containing green fluorescent protein-tagged connexin43. Thus, neither actin filaments nor microtubules appear to restrict GJ assembly by anchoring intramembrane GJ proteins, nor are they absolutely required for functional GJs to form. However, microtubules are necessary for enhanced GJ growth and likely for facilitating connexin trafficking under basal conditions.

Isolation and characterization of XKaiso, a transcriptional repressor that associates with the catenin Xp120(ctn) in Xenopus laevis.

The Armadillo family of catenin proteins function in multiple capacities including cadherin-mediated cell-cell adhesion and nuclear signaling. The newest catenin, p120(ctn), differs from the classical catenins and binds to the membrane-proximal domain of cadherins. Recently, a novel transcription factor Kaiso was found to interact with p120(ctn), suggesting that p120(ctn) also possesses a nuclear function. We isolated the Xenopus homolog of Kaiso, XKaiso, from a Xenopus stage 17 cDNA library. XKaiso contains an amino-terminal BTB/POZ domain and three carboxyl-terminal zinc fingers. The XKaiso transcript was present maternally and expressed throughout early embryonic development. XKaiso's spatial expression was defined via in situ hybridization and was found localized to the brain, eye, ear, branchial arches, and spinal cord. Co-immunoprecipitation of Xenopus p120(ctn) and XKaiso demonstrated their mutual association, whereas related experiments employing differentially epitope-tagged XKaiso constructs suggest that XKaiso additionally self-associates. Finally, reporter assays employing a chimera of XKaiso fused to the GAL4 DNA binding domain indicate that XKaiso is a transcriptional repressor. These data suggest that XKaiso functions throughout development and that its repressor functions may be most apparent in the context of neural tissues. The significance of the XKaiso-p120(ctn) interaction has yet to be determined, but it may include transducing information from cadherin-mediated cell-cell contacts to transcriptional processes within the nucleus.