Internal strain drives spontaneous periodic buckling in collagen and regulates remodeling

Abstract

Fibrillar collagen, an essential structural component of the extracellular matrix, is remarkably resistant to proteolysis, requiring specialized matrix metalloproteinases (MMPs) to initiate its remodeling. In the context of native fibrils, remodeling is poorly understood; MMPs have limited access to cleavage sites and are inhibited by tension on the fibril. Here, single-molecule recordings of fluorescently labeled MMPs reveal cleavage-vulnerable binding regions arrayed periodically at ∼1-μm intervals along collagen fibrils. Binding regions remain periodic even as they migrate on the fibril, indicating a collective process of thermally activated and self-healing defect formation. An internal strain relief model involving reversible structural rearrangements quantitatively reproduces the observed spatial patterning and fluctuations of defects and provides a mechanism for tension-dependent stabilization of fibrillar collagen. This work identifies internal-strain-driven defects that may have general and widespread regulatory functions in selfassembled biological filaments.collagenase | matrix metalloproteinase | single molecule | pattern formation | mechanosensing C ollagen comprises over 30% of the protein mass of the human body and is an abundant structural protein in all animals (1). In its most common form, fibrillar collagen assembles from ∼300-nm polypeptide triple helices (tropocollagen) into highly organized, hierarchical networks that provide the protein scaffolding for the extracellular matrix, tendons, bones, and other load-bearing structures (1-3). The triple helix is highly resistant to proteolysis and collagen degradation requires a specialized class of proteases called matrix metalloproteinases (MMPs) (1, 4-6). Only four of the 23 human MMPs can initiate degradation of fibrillar collagen (6, 7). They cleave all three polypeptide chains of tropocollagen asynchronously at a unique thermally labile site located ∼225 nm from the N terminus (8, 9). MMPs play important physiological roles during development and wound healing, promoting cell motility, angiogenesis, and tissue remodeling (6,(10)(11)(12). MMP activity is regulated through gene expression and through binding of specific tissue inhibitors of metalloproteinase (13,14). In addition, MMP activity is regulated by mechanical stress on fibrillar collagen, which inhibits proteolysis through an unknown mechanism (15-17). Dysregulation of MMP activity is implicated in rheumatoid arthritis, atherosclerosis, tumor progression, and metastasis (13,18,19).Despite the importance of collagen remodeling for human health and disease, the mechanistic details of how MMPs degrade fibrillar collagen are not well established. Physiologically relevant collagen fibrils are heterogeneous and insoluble filamentous protein assemblies that are refractory to conventional biochemical analysis (20), yet a spatially extended substrate permits measurements of MMPs moving on fibrillar collagen through fluorescence correlation spectroscopy (20-22) and, more recently, direct single...

Programming the mechanics of cohesive fiber networks by compression

“…Tissues are able to actively reinforce their structure along the principal load direction. The mechanisms of mechanical reinforcement are generally thought to originate from cellular activity, involving strain-dependent fiber degradation and synthesis 10,11 . However, recent studies suggest that biopolymer networks are inherently adaptive themselves, since they are held together by weak transient bonds 12,13 .…”

New Proposal to Define the Fascial System

“…Its most common form is the collagen fibril, which is made up of tropocollagen units, i.e., polypeptide triple helices of about 300 nm in length. The fibril is highly organized and provides the protein framework for the extracellular matrix (ECM), the tendons, the bones, and for other supporting structures [15]. Collagen fibrils resemble self-assembling nanoscale wires.…”

“…The weakness found in collagen fibrils is the imprint of the perfection of nature. In fact, these periodic imperfections enable the collagen fibril to handle the tension it perceives, adapting continuously: it reshapes itself to perpetuate its function [15]. The subsidence points are highly susceptible to tension variation sites, where metabolic processes important to regulate the remodeling/reshaping of the tissue of the fibrillar structure are initiated.…”

“…The subsidence points are highly susceptible to tension variation sites, where metabolic processes important to regulate the remodeling/reshaping of the tissue of the fibrillar structure are initiated. These imperfections are like ‘Trojan horses' where specialized proteolytic enzymes (matrix metalloproteinases (MMPs)) can more easily enter and start degradation and repair processes faster [15]. …”

Non‐enzymatic cross‐linking of collagen type II fibrils is tuned via osmolality switch

“…There is some evidence that stretching of collagen fibrils decelerates MMPs enzymatic activity 7, 9, 10, 11. This could be due to the 3D orientation of the amino acids that form the cleavage sites in the backbone of collagen fibrils, which might only interact with enzymes in a specific (low stretched) conformation.…”