ABSTRACT VERSICAN/COLLAGEN INTERACTIONS IN TISSUE STRUCTURE AND MECHANICSDongning Chen Rebecca G. Wells Type I collagen is the most abundant structural protein in the extracellular matrix (ECM), forming a dynamic 3D fibrous network that is highly regulated by other ECM components including proteoglycans (PGs) and glycosaminoglycans (GAGs). Matrix PGs, especially the small leucine rich PG (SLRP) subgroup, have been well studied as collagen binding proteins and regulators of fibrillogenesis. However, the impact of the hyalectan subgroup of PGs, particularly versican, on collagen behaviors is not well understood. There is a particular need for understanding the role of versican in the collagen network because of its universal distribution in tissues and its altered expression during collagen-related fibrotic disorders. My aim was to study collagen/versican interactions and to investigate the role of versican in modulating collagen structural and mechanical behaviors. I used solid phase binding assays and the Collagen Toolkit to identify binding sites, and I carried out in vitro turbidity assays combined with fibroblast-derived matrices (FDM) to study fibrillogenesis. Collagen fiber organization was visualized using scanning electron microscopy (SEM), and cell-mediated collagen realignments and contractions were assessed by collagen plug and engineered microtissue assays. Shear rheometry was carried out on collagen gels and liver tissues to evaluate the impact of versican on tissue mechanics. I determined that versican and its V3 isoform bind collagen via the versican G3 domain and collagen R-G-Hydrophobic-O motif, independent of versican GAG residues. Compared to SLRPs and the structurally similar hyalectan aggrecan, versican shows unique effects on multiple collagen behaviors: 1) versican upregulates collagen gelation and promotes the deposition of collagen-rich matrix with aligned fibers; 2) the presence of versican improves fibril fusion into large bundles and forms a looser network; 3) versican improves cell-mediated collagen compaction, alignment and microtissue contraction; 4) versican contributes to collagen gel mechanics by decreasing stiffness and attenuating strain stiffening. In tissues, versican and its GAGs also play a role by downregulating compression stiffening. Thus, versican is a unique regulator of various collagen behaviors and therefore has potential therapeutic value in collagen-related fibroproliferative diseases such as inflammation, fibrosis and cancer.