The extracellular matrix (ECM) is a fibrous, branched network that is assembled by cells from secreted glycoproteins and proteoglycans and provides specific environmental signals to control cell morphology, migration, differentiation, and proliferation. The ECM is required to organize cells into tissues. Deficiencies in matrix composition and architecture correspond with the onset and progression of developmental defects and major diseases including cancer, fibrosis, and skeletal dysplasias. The goal of our research is to uncover novel mechanisms governing the assembly of the ECM protein fibronectin and its role in directing assembly of other ECM proteins to form a definitive matrix. Understanding these mechanisms will help us to determine how tissue-specific variations in matrix structure and organization regulate and modulate cell activities and contribute to developmental and disease processes.
Fibronectin is a ubiquitous adhesive glycoprotein and a major component of the ECM. As a multi-domain protein, fibronectin interacts with integrin receptors to transmit signals into cells while simultaneously interacting with ECM components such as collagens, proteoglycans, and other fibronectins. The latter interactions are essential for assembly of fibronectin into a multimeric fibrillar matrix. Using cell culture-based systems, we have determined the key steps in the assembly of fibronectin fibrils and our work has implicated fibronectin matrix assembly in a variety of biological processes including cell cycle progression, stem cell self-renewal and differentiation, and collagen fibrillogenesis.
The critical final step in fibronectin matrix assembly is the irreversible conversion of nascent fibronectin fibrils into an insoluble network that then supports the ECM incorporation of collagens, matricellular proteins, and growth factors to build a definitive matrix. Interactions with other fibronectin molecules and with heparan sulfate are needed for fibril assembly. Our results suggest a combination of fibronectin conformational changes and clustering of fibronectins near the cell surface are involved in the assembly process. Ongoing research is targeted at understanding the specific molecular interactions required for fibril insolubility with a focus on the role of heparan sulfate proteoglycans.
In culture, insoluble fibronectin matrix serves as a template for initiation of collagen fibrillogenesis. Several type I collagen processing enzymes bind directly to fibronectin supporting the idea that fibronectin matrix forms a collagen processing hub required for collagen assembly. This role could be particularly critical in very early skeletal development where fibronectin and type I collagen are prevalent. The recent discovery by our collaborators of a mutation in human FN1 in an individual with clinical skeletal dysplasia provides us with a new model to study the role of fibronectin in the skeleton. The identification of this mutation opens entirely new and exciting avenues for research into the effects of fibronectin matrix perturbations on morphogenesis and differentiation. Defects in matrix assembly also lead to fibrosis, a common complication in a metabolic disease such as diabetic nephropathy. Elevated glucose levels activate intracellular pathways, change gene expression patterns, and cause the accumulation of disorganized ECM. We are investigating the results of RNA-seq experiments showing that ECM-related genes are significantly up-regulated by glucose and highlighting pathways not previously linked to ECM that may be contributing to the fibrotic response.
Since assembly of an appropriate ECM is essential for tissue repair, we are developing novel biomaterials using synthetic polymeric materials combined with natural ECM and chemically patterned materials to direct the organization of the ECM. These biomaterials are designed for potential use as tissue mimetics in regenerative medicine. Our work thus far is providing new ideas about how to direct nerve regeneration using a composite ECM-synthetic polymer material as an implant for spinal cord and peripheral nerve repair.