Alpha 7 beta 1 integrin receptor in vascular and visceral smooth muscle cells. Ultrastrucural alterations in alpha 7 deficient mice and compensatory overexpression of alpha 3 chain

Introduction Alpha 7 beta 1 is major integrin receptor for laminin in muscular tissue. The absence of this integrin correlates wtih skeletal muscular distrophy (Mayer U. et al, Nature Genet,1997,17,318-23). Severe tissue alterations have been also found in deficient mouse myocardium (Ortolani F. et al, in press). To date, little attention has been focussed on smooth muscle cells, in spite of the strong expression of alpha 7 integrin chain in these cells. Material Specimens were excised from the muscularis externa of the walls of oesophagus, stomach, ileus, colon, bladder and uterus of normal and alpha 7 deficient mice. In addition, comparison between wilde type and knock-out animals was carried out for smooth muscle cells forming the tunica media of vessels irrorating myocardium, different skeletal muscles and the walls of the above mentioned viscera Antibodies rabbit polyclonal R242 antibody against murine alpha7 integrin chain, rabbit anti-integrin alpha3 chain polyclonal antibody (Chemicon International-Prodotti Gianni, Milan); rabbit anti-integrin alpha5 chain polyclonal antibody (Chemicon International-Prodotti Gianni, Milan); rat anti-integrin alpha6 chain (GoH3) monoclonal antibody (Chemicon International-Prodotti Gianni, Milan). Results Specific alterations were observed in deficient smooth muscle cells, including (i) enlargement of plasma membranes, which showed a lot of indentations particularly marked at the interface with the extracellular matrix, (ii) prominent arising of caveolae and intracytoplasmatic vesicles, and (iii) significant increasing in the distribution of organules and subsarcolemmal/sarcomeric dense bodies. These alterations were more marked in vascular walls than in visceral ones. Immunofluorescence and immunogold labelling revealed no differences to exist concerning the expression of integrins alpha 5 and alpha 6 in mutant tissues versus normal ones. Conversely, increase in alpha 3 integrin chain expression was detected in deficient smooth muscle cells, which was more evident in visceral walls than in vascular ones. Conclusions These results confirm the important role of alpha 7 beta 1 integrin in ensuring muscle tissue structural integrity , especially there where mechanical stress is stronger. The most severe alterations exhibited by vascular smooth muscle cells might correlate with the uninterrupted stress exerted by the hemodynamical flow on vessel walls and with the less efficient compensatory effect of alpha 3 integrin chain, since alpha 3 seems to undergo weaker up-regulation for mutant muscle cells in vessels than for those in viscera.