Matrix Metalloproteinase-9 Deficiency Increases Expression Of Aortic Matrix Precursors

Amy E Hackmann, Batool Arif, Monica B. Pagano, Terri L. Ennis, Robert W. Thompson, John A. Curci.
Washington University School of Medicine, St. Louis, MO.

OBJECTIVES: Aortic dilatation after elastase perfusion results from inflammation and matrix proteolytic activity, but may also be related to impaired matrix repair processes. Matrix metalloproteinase (MMP)-9 is critical to aneurysm development in this model, and deletion of the enzyme results in aneurysm-resistant mice. By studying the expression of precursors of critical matrix proteins, alpha-1 procollagen (PC) and tropoelastin (TE), as well as the enzyme necessary for collagen and elastin cross-linking, lysyl oxidase (LO), we evaluated the matrix repair response of the aorta after injury in MMP-9 -/- mice.

METHODS: Male wild-type and MMP-9 -/- SvEv mice underwent infrarenal aortic perfusion with a dilute elastase solution. At either two or eight weeks postoperatively, the mice were sacrificed and the perfused segments of aorta were measured in vivo and processed for mRNA expression by RT-PCR. Lysyl oxidase, procollagen, and tropoelastin levels were measured and normalized to GAPDH.

RESULTS: Wild-type mice demonstrated an increase in aortic diameter of 116+5.0% at two weeks and 110+5.7% at eight weeks. MMP-9 -/- mice had only a 66+3.7% increase at 2 weeks and a 77+4.6% at eight weeks. In the MMP-9 -/- mice there were greater levels of LO (2.94+.37 vs. .24+.04, p<.0001), PC (8.81+1.67 vs. 1.20+.19, p=.0027), and especially TE (284.51+45.70 vs. 2.09+.59, p=.0003) at eight weeks compared to two weeks. More significantly, the knockout mice exhibited higher expression of PC (1.20+.19 vs. .53+.11, p=.0054) and TE (2.09+.59 vs. .90+.26, p=.0504) at two weeks and LO (2.94+.37 vs. .59+.11, p<.0001), PC (8.81+1.67 vs. .96+.21, p=.0002), and TE (284.51+45.70 vs. 1.89+.71, p<.0001), which is two orders of magnitude larger, at eight weeks when compared to aortas from the wild-type animals. The expression of all three genes in the aorta remains stable in the wild type mice between the two time points.

CONCLUSIONS: The removal of the elastase, MMP-9, prevents the development of model AAA and apparently preserves medial elastin. The effect may, at least in part, be mediated by enhanced medial matrix repair through mechanisms which have yet to be defined.