Super-Resolution Mapping of the Dynamics of Periodic Structural Defects in Collagen Fibrils

Abstract

Single-molecule tracking of matrix metalloproteinases (MMPs) moving on fibrillar collagen reveals a regular binding pattern with a 1.1 µm periodicity. The binding sites exhibit collective motion that preserves the distribution but not the phase of the binding pattern. Over short timescales (∼1 s), the motion of individual binding sites is consistent with diffusion in a harmonic potential. At longer timescales (∼20 s), the potential wells slowly migrate, bifurcate, and merge. The dynamic nature of the binding sites suggests that they correspond to transient local defects in the collagen fibril structure. However, the long-range order of their pattern, exceeding any known structural scale of the fibril, indicates a collective defect formation process. We propose a model in which internal strain energy in fibrillar collagen is relieved by the formation of defects that are distributed along its length. This model falls into the general class of mechanical instabilities that generate long-range spatial patterning in physical systems ranging from mud cracking to skin wrinkling. However, unlike cracks and wrinkles that are stable structures, the microscopic fibril features thermally excited structural dynamics and self-healing of defect states. One physiological consequence of the proposed model is that external tension opposing the internal strain in the fibril can suppress defect formation and exposure of the MMP binding sites. Experiments showing that external loading attenuates the enzymatic degradation of fibrillar collagen are consistent with this prediction of the model. More generally, many aspects of collagen degradation, including cleavage initiation, processivity, and kinetics, may largely be a consequence of a previously unrecognized structural heterogeneity in the underlying fibrillar substrate. Thus, mapping the periodic array of defects in the molecular architecture of collagen elucidates a key feature regulating enzymatic activity and remodeling of the extracellular matrix.