We have previously reported a unique response of traction force generation for 
cells grown on mature cardiac ECM, where traction force was constant over a 
range of stiffnesses. In this study we sought to further investigate the role of 
the complex mixture of ECM on this response and assess the potential mechanism 
behind it. Using traction force microscopy, we measured cellular traction forces 
and stresses for mesenchymal stem cells (MSCs) grown on polyacrylamide gels at a 
range of stiffnesses (9, 25, or 48 kPa) containing either adult rat heart ECM, 
different singular ECM proteins including collagen I, fibronectin, and laminin, 
or ECM mimics comprised of varying amounts of collagen I, fibronectin, and 
laminin. We also measured the expression of integrins on these different 
substrates as well as probed for β1 integrin binding. There was no significant 
change in traction force generation for cells grown on the adult ECM, as 
previously reported, whereas cells grown on singular ECM protein substrates had 
increased traction force generation with an increase in substrate stiffness. 
Cells grown on ECM mimics containing collagen I, fibronectin and laminin were 
found to be reminiscent of the traction forces generated by cells grown on 
native ECM. Integrin expression generally increased with increasing stiffness 
except for the β1 integrin, potentially implicating it as playing a role in the 
response to adult cardiac ECM. We inhibited binding through the β1 integrin on 
cells grown on the adult ECM and found that the inhibition of β1 binding led to 
a return to the typical response of increasing traction force generation with 
increasing stiffness. Our data demonstrates that cells grown on the mature 
cardiac ECM are able to circumvent typical stiffness related cellular behaviors, 
likely through β1 integrin binding to the complex composition.