Collagens are extremely important extracellular matrix (ECM) proteins that play dual roles as structural scaffolds, maintaining tensile strength of connective tissues throughout the body, and as biologically active proteins, interacting with numerous receptors, enzymes, and other ECM components to carry out critical functions, such as ECM adhesion, differentiation, and hemostasis. Many common diseases, such as arthritis, diabetes, and cancer involve abnormal regulation or reactivity of collagen, and certain debilitating heritable connective tissue diseases arise from collagen mutations. In the ECM, collagens self-assemble into higher order structures, such as fibrils and non-fibrillar networks.
The Baum lab is interested in gaining a molecular understanding of how the surface properties of collagen fibrils in the ECM facilitate their biomolecular interactions with integrins, matrix metalloproteinases, and polysaccharides. We are especially interested in how modifications to the ECM environment within tissues, as occurs with aging, or substitutions in the collagen amino acid sequence, as in heritable connective tissue diseases (e.g. Osteogenesis Imperfecta or Ehlers-Danlos Syndromes), alter the biophysical properties of the collagen fibrils and their assembly mechanism. Despite its crucial biological importance, very little is understood about the molecular level impact of these modifications on collagen structure and function in the ECM. We are bridging this knowledge gap by pursuing a multifaceted biophysical study on full-length collagen in ECM secreted from osteoblasts in systems that have been modified to model heritable connective tissue diseases or the aging environment. We integrate high-field solution and solid-state NMR with TEM, AFM, cell adhesion and cell receptor binding assays, and computational methods to gain a molecular level understanding of the impact of collagen sequence modifications or ECM environment alterations on collagen structure and function.
Collagen mutations associated with disease:
Although the collagen triple helix has a relatively uniform rod-like structure, with an amino acid sequence of (Gly-X-Y)n repeats, subtle alterations in structure and dynamics along the collagen chain are critical to many biological events.The substitution of a single Gly by another amino acid breaks the characteristic repeating (Gly-X-Y)n sequence pattern of collagen and results in connective tissue disease. Gly mutations in collagen type I in bone, tendons, and skin result in Osteogenesis Imperfecta (OI), which is characterized by brittle bones with phenotypes ranging from non-lethal to lethal. Mutations to collagen type III in blood vessels and hollow organs result in vascular Ehlers-Danlos Syndrome (vEDS), the most detrimental of EDS genetic connective tissue diseases, and lead to arterial, intenstinal, and uterine rupture. We ask: How do detrimental G-->X mutations impact collagen triple helix structure, dynamics, stability, and protein binding in order to establish a sequence-structure/dynamics-biological function relationship to understand the molecular basis of disease.
Collagen–protein interactions regulate biological function:
Collagen is highly biologically active, interacting with cellular receptors, such as integrins, matrix metalloproteinases (MMPs) during degradation, and other ECM components. These collagen interactions are critical for biological function, however in other cases, improper interactions with collagen may be detrimental. Our lab studies collagen binding to integrin receptors, α1β1 and α2β1, through their α1 and α2 I-domains. Specific binding of integrins to collagen plays a critical role in numerous cellular adhesion processes including platelet activation and aggregation, a key process in clot formation, and cancer metastasis. In our research, we ask: How does the integrin I-domain select, recognize and bind collagen? The interaction of collagen with β2-microglobulin (β2m) is pathological and results in β2m amyloid fibrils in the joints of long-term hemodialysis patients, termed dialysis-related amyloidosis. We ask: How is β2m structurally and dynamically perturbed by its interaction with collagen to promote amyloid formation?
Understanding the complex architecture of the collagen fibril for access to cryptic binding sites:
Triple helical monomers of fibrillar collagens pack into a complex, supramolecular fibril architecture. Although in the long, rod-like triple helix, all interaction sites are exposed and available for binding, the packing of the fibril obstructs many of these interactions sites, hiding them from the exposed fibril surface. Since critical biological processes requiring access to binding sites still occur, such as cellular attachment and MMP cleavage, we ask: How does the complex collagen architecture regulate protein binding? To answer this question, we probe the internal dynamics of the collagen type I fibril by molecular dynamics simulations and use microscopy to visualize collagen fibrils and monomers and their binding partners.