The role of MMP-I up-regulation in the increased compliance in muscle-derived stem cell-seeded small intestinal submucosa

We have previously observed that muscle-derived stem cells (MDSC) seeded onto porcine small intestinal submucosa (SIS) increase the mechanical compliance of the engineered tissue construct [Lu SH, Sacks MS, Chung SY, Gloeckner DC, Pruchnic R, Huard J, et al. Biaxial mechanical properties of muscle-derived cell seeded small intestinal submucosa for bladder wall reconstitution. Biomaterials 2005;26(4):443–9]. To date, however, the initial remodeling events which occur when MDSC are seeded onto SIS have yet to be elucidated. One potential mechanism responsible for the observed increase in mechanical compliance is the release of matrix metalloproteinase-I (MMP-I). To investigate this finding, MDSC (∼1×106) were cultured on single-layer SIS cell culture inserts (4.7 cm2) for 1–10 days. MDSC MMP-I activity on SIS in the supernatant at 1, 3, 5, 7, and 10 days was determined using a collagenase assay kit. MMP-I activity of the MDSC/SIS was significantly higher (p<0.0025) after one day in culture compared to specimens collected from subsequent time points and the unseeded control. To further study the initial remodeling events, the impact of MMP-I on mechanical compliance was examined. SIS was incubated with 0.16 U/mL collagenase-I for 3, 4.5, 5, and 24 h, then biaxial mechanical testing was performed. After 5 h of digestion with collagenase-I, mechanical compliance under 1 MPa peak stress was increased by 7% in the circumferential direction, compared to control SIS. These findings suggest that the release of MMP-I in response to initial seeding on SIS and subsequent breakdown of collagen fibers is the mechanism responsible for an increase in mechanical compliance.