Mapping transmembrane binding partners for E-cadherin ectodomains.

We combine proximity labeling and single molecule binding assays to discover transmembrane protein interactions in cells. We first screen for candidate binding partners by tagging the extracellular and cytoplasmic regions of a "bait" protein with BioID biotin ligase and identify proximal proteins that are biotin tagged on both their extracellular and intracellular regions. We then test direct binding interactions between proximal proteins and the bait, using single molecule atomic force microscope binding assays. Using this approach, we identify binding partners for the extracellular region of E-cadherin, an essential cell-cell adhesion protein. We show that the desmosomal proteins desmoglein-2 and desmocollin-3, the focal adhesion protein integrin-α2ß1, the receptor tyrosine kinase ligand ephrin-B1, and the classical cadherin P-cadherin, all directly interact with E-cadherin ectodomains. Our data shows that combining extracellular and cytoplasmic proximal tagging with a biophysical binding assay increases the precision with which transmembrane ectodomain interactors can be identified.

The area composita of adhering junctions connecting heart muscle cells of vertebrates. VII. The different types of lateral junctions between the special cardiomyocytes of the conduction system of ovine and bovine hearts.

Postnatal development of mammalian cardiomyocytes in the working myocardium is characterized by a near-complete translocation of both kinds of adhering junctions (AJs), i.e. desmosomes and fasciae adhaerentes (FAs), to the polar intercalated disk (ID) regions where they cluster, fuse and molecularly amalgamate to extended hybrid intercellular junction structures, the area composita (composite junction; AC). Using immunofluorescence and immunoelectron microscopy we now report that the AJ structures of the conduction system, in particular those of the Purkinje fiber cells of cows and sheep are fundamentally different. Here the numerous AJs remain in lateral connections with other conductive cells. Desmosomal or desmosome-like junctions can still be distinguished from FA junctions, and a third type of AJs can be identified which shows colocalization of desmosomal and FA proteins, i.e. an AC character. These results, together with demonstrations of other cell type cytoskeletal markers such as alpha-cardiac actin and desmin, support the concept that conductive cells are derived from embryonal cardiomyocytes and are arrested at an early stage of differentiation. We also show that the conductive cells have extended plasma membrane regions characterized by an exceptionally high proportion of junctions with desmosomal character and proteins, amounting to 50% and more, resulting in the highest desmosome protein packing so far described in non-epithelial cells. The relevance of these junctions for the formation, maintenance and functions of the conductive system is discussed, together with the conclusion that the desmosome-rich regions of conductive cells are among the most vulnerable sites for functional disorders caused by desmosomal protein mutations.

Effects of hyperosmotic stress on cultured airway epithelial cells.

Inhalation of hyperosmotic solutions (salt, mannitol) has been used in the treatment of patients with cystic fibrosis or asthma, but the mechanism behind the effect of hyperosmotic solutions is unclear. The relation between osmolarity and permeability changes was examined in an airway cell line by the addition of NaCl, NaBr, LiCl, mannitol, or xylitol (295-700 mOsm). Transepithelial resistance was measured as an indicator of the tightness of the cultures. Cell-cell contacts and morphology were investigated by immunofluorescence and by transmission electron microscopy, with lanthanum nitrate added to the luminal side of the epithelium to investigate tight junction permeability. The electrolyte solutions caused a significant decrease in transepithelial resistance from 450 mOsm upwards, when the hyperosmolar exposure was gradually increased from 295 to 700 mOsm; whereas the nonelectrolyte solutions caused a decrease in transepithelial resistance from 700 mOsm upwards. Old cultures reacted in a more rigid way compared to young cultures. Immuno-fluorescence pictures showed weaker staining for the proteins ZO-1, claudin-4, and plakoglobin in treated samples compared to the control. The ultrastructure revealed an increased number of open tight junctions as well as a disturbed morphology with increasing osmolarity, and electrolyte solutions opened a larger proportion of tight junctions than nonelectrolyte solutions. This study shows that hyperosmotic solutions cause the opening of tight junctions, which may increase the permeability of the paracellular pathway and result in increased transepithelial water transport.

The area composita of adhering junctions connecting heart muscle cells of vertebrates - III: assembly and disintegration of intercalated disks in rat cardiomyocytes growing in culture.

For cell and molecular biological studies of heart formation and function cell cultures of embryonal, neonatal or adult hearts of various vertebrates, notably rat and chicken, have been widely used. As the myocardium-specific cell-cell junctions, the intercalated disks (ID), have recently been found to be particularly sensitive to losses of - or mutations in - certain cytoskeletal proteins, resulting in cardiac damages, we have examined the ID organization in primary cultures of cardiomyocytes obtained from neonatal rats. Using immunofluorescence and immunoelectron microscopy, we have studied the major ID components for up to 2 weeks in culture, paying special attention to spontaneously beating, individual cardiomyocytes and myocardial cell colonies. While our results demonstrate the formation of some ID-like cardiomyocyte-connecting junction arrays, they also reveal a variety of structural disorders such as rather extended, junction-free ID regions, sac-like invaginations and endocytotic blebs as well as accumulations of intracytoplasmic structures suggestive of endocytosed forms of junction-derived vesicles or of junction fragments resembling fascia adhaerens elements. Moreover, we have noticed a novel type of small, obviously plaque-free cytoplasmic vesicles containing one or both of the desmosomal cadherins, desmocollin Dsc2 and desmoglein Dsg2. We conclude that cardiomyocyte cultures are useful model systems for studies of certain aspects of myocardiac differentiation and functions but, on the other hand, show progressive disintegration and deterioration. The potential value of molecular markers and reagents in studies of myocardial pathology as well as in the monitoring of myocardial differentiation of so-called stem cells is discussed.