shRNA construct against matrix metalloproteinase 2 (MMP-2) (shMMP2) inhibits strengthening of collagen gels by annulus fibrosus (AF) cells. Parameters obtained from mechanical testing of collagen gels. (a) Rupture stress of collagen gels seeded with wild-type noninfected cells was significantly higher than that of acellular gels, gels populated with shNon-transduced cells, and gels populated with shMMP2-transduced cells (P < 0.05). (b) Short-term and (c) long-term moduli were lower in cell-seeded collagen gels but not significantly different from acellular gels. (d) Collagen gels seeded with cells possessed significantly lower viscous coefficients than did acellular gels (P < 0.05).