Increased expression of α1-integrin is sufficient to mediate adaptation of neurons to aggrecan.A, Neurons infected with adenoviral constructs expressing a control protein (β-Gal,right) or mouse α1-integrin (left) were stained live with an antibody that specifically recognizes the mouse α1 subunit.Arrows indicate the positions of neuronal cells.B, Overexpression of mouse α1-integrin in chick neurons results in an approximately twofold increase in total α1-integrin expressed at the cell surface (2.25 + 0.2). Infected neurons were cultured on LM substrata, where endogenous integrin expression is low (see Materials and Methods). Cell surface-biotinylated proteins from mouse α1-integrin-infected (lane 1) or control, β-galactosidase-infected (lane 2) neurons were immunoprecipitated with an antisera that recognizes both mouse and chick α1-integrin subunits. Total α1-integrin expressed at the surface is increased after infection with α1-expressing adenovirus, reflecting the contribution of the exogenous mouse protein to total α1 expression. Expression of endogenous α1 (control cells, lane 2) is quite low under these conditions. C, Control or α1-integrin-infected neurons were cultured on substrata containing laminin alone or laminin in combination with aggrecan as in Figure 4. Control neurons expressing β-galactosidase (black bars) are strongly inhibited in the presence of aggrecan after 3 hr in culture (Figs. 1, 4). At this time in culture, endogenous increases in integrin expression have not yet occurred. Neurons with increased laminin-receptor expression (α1-expressing; white bars) are not inhibited by aggrecan, showing outgrowth equivalent to that seen on laminin alone. Means + SEMs from at least three experiments are given. *Significantly different from all other conditions at p < 0.0001 (ttest).