MMP9 and pulmonary emphysema: To test the relative contribution of MMP-9 vs. NE in emphysema formation, we complemented our studies of distal airspace morphometry with invasive pulmonary function testing in βENaC-Tg mice lacking either MMP-9 (βENaC-Tg/Mmp9-/-) or NE (βENaC-Tg/NE-/-) (Figure 6).