N-CADHERIN IN ADULT-RAT CARDIOMYOCYTES IN CULTURE .2. SPATIOTEMPORAL APPEARANCE OF PROTEINS INVOLVED IN CELL-CELL CONTACT AND COMMUNICATION- FORMATION OF 2 DISTINCT N-CADHERIN CATENIN COMPLEXES/

The spatio-temporal appearance and distribution of proteins forming th e intercalated disc were investigated in adult rat cardiomyocytes (ARC ), The 'redifferentiation model' of ARC involves extensive remodelling of the plasma membrane and of the myofibrillar apparatus, It represen ts a valuable system to elucidate the formation of cell-cell contact b etween cardiomyocytes and to assess the mechanism by which different p roteins involved in the cell-cell adhesion process are sorted in a pre cise manner to the sites of function, Appearance of N-cadherin, the ca tenins and connexin43 within newly formed adherens and gap junctions w as studied. Here first evidence is provided for a formation of two dis tinct and separable N-cadherin/catenin complexes in cardiomyocytes, Bo th complexes are composed of N-cadherin and alpha-catenin which bind t o either beta-catenin or plakoglobin in a mutually exclusive manner. T he two N-cadherin/catenin complexes are assumed to be functionally inv olved in the formation of cell-cell contacts in ARC; however, the diff erential appearance and localization of the two types of complexes may also point to a specific role during ARC differentiation. The newly s ynthesized beta-catenin containing complex is more abundant during the first stages in culture after ARC isolation, while the newly synthesi zed plakoglobin containing complex progressively accumulates during th e morphological changes of ARC. ARC formed a tissue-like pattern in cu lture whereby the new cell-cell contacts could be dissolved through Ca 2+ depletion, Presence of cAMP and replenishment of Ca2+ content in th e culture medium not only allowed reformation of cell-cell contacts bu t also affected the relative protein ratio between the two N-cadherin/ catenin complexes, increasing the relative amount of newly synthesized beta-catenin over plakoglobin at a particular stage of ARC differenti ation. The clustered N-cadherin/catenin complexes at the plasma membra ne appear to be a prerequisite for the following gap junction formatio n; a temporal sequence of the appearance of adherens junction proteins and of gap junctions forming connexin43 is suggested.