Expression and Cellular Distribution of αvIntegrins inβ1 Integrin-deficient Embryonic Stem Cell-derived Cardiac Cells

β1integrin-deficient (β1−/−) ES cells showed increased differentiation of cardiac cells characterized by reduced adhesion and high beating frequency. Whereas in whole embryoid body outgrowths of β1−/−cells maximum levels ofαv , β3and β5integrin mRNA were delayed and transiently upregulated, in cardiac clusters isolated from β1−/−cells, onlyβ3 integrin mRNA levels were enhanced in comparison to wild-type (wt) cells. To answer the question, whetherαv and β3integrins may compensate, at least partially, the loss ofβ1 integrin function during cardiac differentiation, the distribution of αvandβ3 integrins in β1−/−and wt pacemaker-like cardiac cells was analyzed. A different distribution of αvand β3integrins inβ1−/−v wt cardiac cells was found. In wt cardiac cells,β1 integrin was localized in specialized subsarcolemmal regions, in particular, at focal contacts and costameres, but αvintegrin was diffusely distributed. In contrast, inβ1−/− cardiac cells, αvintegrin was preponderantly localized at cell membranes, focal contacts and costameres. β3integrin displayed a diffuse pattern both in wt and in β1−/−pacemaker-like cells at early differentiation stages, whereas at terminal stages, β3was colocalized with sarcomeres in wt, but not inβ1−/− pacemaker-like cells. Quantitative immunofluorescence analysis revealed increased αvand β3integrin levels inβ1−/− pacemaker-like cardiac cells. Our results led us to conclude that altered cellular distribution of αvintegrin and upregulation of β3integrin correlate with growth and survival of β1−/−cardiac pacemaker-like cells at an early developmental state. However, αvand β3integrins cannot functionally compensate the loss of β1integrin during terminal differentiation of cardiac cells implicating that cardiomyocytes require specific β1integrin functions for cardiac specialization.