267 TISSUE ENGINEERING Volume 11, Number 1/2, 2005 © Mary Ann Liebert, Inc. Matrix Metalloproteinase 9 Facilitates Collagen Remodeling and Angiogenesis for Vascular Constructs HAK-JOON SUNG, Ph.D., 1,2 CHAD E. JOHNSON, Ph.D., 1 SUSAN M. LESSNER, Ph.D., 1 RICHARD MAGID, Ph.D., 1 DANIELLE N. DRURY, B.S., 1 and ZORINA S. GALIS, Ph.D. 1 ABSTRACT Degradation of the extracellular matrix, facilitated by matrix metalloproteinases (MMPs), can lead to mechanical failure of vascular constructs, suggesting that MMP inhibition could improve sur- vival of constructs. Therefore, we investigated the role of MMP-9 in collagen remodeling in vitro, focusing on the three major steps of production, degradation, and organization. Because an ade- quate blood supply is essential for survival of tissue-engineered constructs, we also evaluated the in- fluence of MMP-9 deficiency on angiogenesis in vivo by implantation of thin biodegradable polymer scaffolds. Using aortic smooth muscle cells (SMCs) from wild-type and genetically deficient (9KO) mice, we examined the role of MMP-9 in collagen mRNA expression and protein accumulation, both with and without ascorbic acid treatment. We measured collagen assembly in a fibrillogenesis as- say. We investigated in vivo angiogenesis and cell invasion, using fluorescence microangiography and histology. MMP-9 deficiency did not affect collagen mRNA production or polymer scaffold degradation, but collagen accumulation was greater in cultures of 9KO SMCs than in wild-type SMCs. Both MMP-9 deficiency and chemical inhibition impaired collagen degradation. Ascorbic acid treatment enhanced collagen production by 9KO SMCs compared with wild-type SMCs at 3 days, but by 7 days this effect was reversed. MMP-9 improved fibrillogenesis of collagen, signifi- cantly more on ascorbic acid treatment. MMP-9 deficiency dramatically decreased inflammatory cell invasion, but also capillary formation within biodegradable polymer scaffolds in vivo. Our data suggest that MMP inhibition, by impairing collagen organization and angiogenesis, might have detri- mental effects on the survival of vascular constructs. 1 Department of Biomedical Engineering, Georgia Institute of Technology/Emory University, Atlanta, Georgia. 2 Division of Cardiology, Emory University School of Medicine, Atlanta, Georgia. INTRODUCTION W E PREVIOUSLY DEFINED vascular remodeling as any enduring change in the size and/or composition of an adult blood vessel, allowing adaptation and repair. On the other hand, inappropriate remodeling, including its absence, underlies the pathogenesis of major car- diovascular diseases, such as atherosclerosis and restenosis. Physiological and pathological vascular re- modeling entails degradation, production, and reorga- nization of the extracellular matrix (ECM) of the ves- sel wall, explaining the interest of investigators in the potential participation of specialized enzymes, called matrix metalloproteinases (MMPs). 1 Mechanical stim- ulation of cell-seeded collagen gels induced further re- modeling, resulting in enhanced mechanical proper- ties. 2,3 However, it has been reported that the initial improvement of mechanical properties was ultimately lost, which may be analogous to the failure of native ar- teries, where vascular pathologies have been associated