Collagen Reorientation Research Articles

Contribution of collagen degradation and proteoglycan depletion to cartilage degeneration in primary and secondary osteoarthritis: an in silico study

Current experimental approaches cannot elucidate the effect of maladaptive changes on the main cartilage constituents during the degeneration process in osteoarthritis (OA). In silico approaches, however, allow creating 'virtual knock-out' cases to elucidate these effects in a constituent-specific manner. We used such an approach to study the main mechanisms of cartilage degeneration in different mechanical loadings associated with the following OA etiologies: (1) physiological loading of degenerated cartilage, (2) injurious loading of healthy intact cartilage and (3) physiological loading of cartilage with a focal defect. We used the recently developed Cartilage Adaptive REorientation Degeneration (CARED) framework to simulate cartilage degeneration associated with primary and secondary OA (OA cases (1)-(3)). CARED incorporates numerical description of tissue-level cartilage degeneration mechanisms in OA, namely, collagen degradation, collagen reorientation, fixed charged density loss and tissue hydration increase following mechanical loading. We created 'virtual knock-out' scenarios by deactivating these degenerative processes one at a time in each of the three OA cases. In the injurious loading of intact and physiological loading of degenerated cartilage, collagen degradation drives degenerative changes through fixed charge density loss and tissue hydration rise. In contrast, the two later mechanisms were more prominent in the focal defect cartilage model. The virtual knock-out models reveal that injurious loading to intact cartilage and physiological loading to degenerated cartilage induce initial degenerative changes in the collagen network, whereas, in the presence of a focal cartilage defect, mechanical loading initially causes proteoglycans (PG) depletion, before changes in the collagen fibril network occur.

An in silico Framework of Cartilage Degeneration That Integrates Fibril Reorientation and Degradation Along With Altered Hydration and Fixed Charge Density Loss.

Injurious mechanical loading of articular cartilage and associated lesions compromise the mechanical and structural integrity of joints and contribute to the onset and progression of cartilage degeneration leading to osteoarthritis (OA). Despite extensive in vitro and in vivo research, it remains unclear how the changes in cartilage composition and structure that occur during cartilage degeneration after injury, interact. Recently, in silico techniques provide a unique integrated platform to investigate the causal mechanisms by which the local mechanical environment of injured cartilage drives cartilage degeneration. Here, we introduce a novel integrated Cartilage Adaptive REorientation Degeneration (CARED) algorithm to predict the interaction between degenerative variations in main cartilage constituents, namely collagen fibril disorganization and degradation, proteoglycan (PG) loss, and change in water content. The algorithm iteratively interacts with a finite element (FE) model of a cartilage explant, with and without variable depth to full-thickness defects. In these FE models, intact and injured explants were subjected to normal (2 MPa unconfined compression in 0.1 s) and injurious mechanical loading (4 MPa unconfined compression in 0.1 s). Depending on the mechanical response of the FE model, the collagen fibril orientation and density, PG and water content were iteratively updated. In the CARED model, fixed charge density (FCD) loss and increased water content were related to decrease in PG content. Our model predictions were consistent with earlier experimental studies. In the intact explant model, minimal degenerative changes were observed under normal loading, while the injurious loading caused a reorientation of collagen fibrils toward the direction perpendicular to the surface, intense collagen degradation at the surface, and intense PG loss in the superficial and middle zones. In the injured explant models, normal loading induced intense collagen degradation, collagen reorientation, and PG depletion both on the surface and around the lesion. Our results confirm that the cartilage lesion depth is a crucial parameter affecting tissue degeneration, even under physiological loading conditions. The results suggest that potential fibril reorientation might prevent or slow down fibril degradation under conditions in which the tissue mechanical homeostasis is perturbed like the presence of defects or injurious loading.

The influence of fiber dispersion on the mechanical response of aortic tissues in health and disease: a computational study

Changes in the structural components of aortic tissues have been shown to play a significant role in the pathogenesis of aortic degeneration. Therefore, reliable stress analyses require a suitable and meaningful constitutive model that captures micro-structural changes. As recent data show, in-plane and out-of-plane collagen fiber dispersions vary significantly between healthy and aneurysmatic aortic walls. The aim of this study is to computationally investigate the influence of fiber dispersion on the mechanical response of aortic tissues in health and disease. In particular, the influence of three different fiber dispersions is studied: (i) non-rotationally symmetric dispersion, the most realistic assumption for aortic tissues; (ii) transversely isotropic dispersion, a special case; (iii) perfectly aligned fibers (no dispersion in either plane), another special case. Explicit expressions for the stress and elasticity tensors as needed for the implementation in a finite element code are provided. Three representative numerical examples are studied: planar biaxial extension, inflation of residually stressed and pre-stretched aortic segments and inflation of an idealized abdominal aortic aneurysm (AAA) geometry. For the AAA geometry the case of isotropic dispersion is additionally analyzed. Documented structural and mechanical parameters are taken from human aortas (healthy media/adventitia and AAA). The influence of fiber dispersions upon magnitudes and distributions of stresses and deformations are presented and analyzed. Stresses vary significantly, especially in the AAA case, where material stiffening is significantly influenced by fiber dispersion. The results highlight the need to incorporate the structural differences into finite element simulations to obtain more accurate stress predictions. Additionally, results show the capability of one constitutive model to represent different scenarios of aortic micro-structures allowing future studies of collagen reorientation during disease progression.

Acellular dermal matrix collagen responds to strain by intermolecular spacing contraction with fibril extension and rearrangement.

Acellular dermal matrix (ADM) materials are used as scaffold materials in reconstructive surgery. The internal structural response of these materials in load-bearing clinical applications is not well understood. Bovine ADM is characterized by small-angle X-ray scattering while subjected to strain. Changes in collagen fibril orientation (O), degree of orientation as an orientation index (OI) (measured both edge-on and flat-on to the ADM), extension (from d-spacing changes) and changes to intermolecular spacing are measured as a result of the strain and stress in conjunction with mechanical measurements. As is already well established in similar systems, when strained, collagen fibrils in ADM can accommodate the strain by reorienting by up to 50° (as an average of all the fibrils). This reorientation corresponds to the OI increasing from 0.3 to 0.7. Here it is shown that concurrently, the intermolecular spacing between tropocollagen decreases by 10% from 15.8 to 14.3Å, with the fibril diameter decreasing from 400 to 375Å, and the individual fibrils extending by an average of 3.1% (D-spacing from 63.9 to 65.9nm). ADM materials can withstand large strain and high stress due to the combined mechanisms of collagen reorientation, individual fibril extension, sliding and changes in the molecular packing density.

A computational algorithm to simulate disorganization of collagen network in injured articular cartilage.

Cartilage defects are a known risk factor for osteoarthritis. Estimation of structural changes in these defects could help us to identify high risk defects and thus to identify patients that are susceptible for the onset and progression of osteoarthritis. Here, we present an algorithm combined with computational modeling to simulate the disorganization of collagen fibril network in injured cartilage. Several potential triggers for collagen disorganization were tested in the algorithm following the assumption that disorganization is dependent on the mechanical stimulus of the tissue. We found that tensile tissue stimulus alone was unable to preserve collagen architecture in intact cartilage as collagen network reoriented throughout the cartilage thickness. However, when collagen reorientation was based on both tensile tissue stimulus and tensile collagen fibril strains or stresses, the collagen network architecture was preserved in intact cartilage. Using the same approach, substantial collagen reorientation was predicted locally near the cartilage defect and particularly at the cartilage-bone interface. The developed algorithm was able to predict similar structural findings reported in the literature that are associated with experimentally observed remodeling in articular cartilage. The proposed algorithm, if further validated, could help to predict structural changes in articular cartilage following post-traumatic injury potentially advancing to impaired cartilage function.

Estrogen-induced collagen reorientation correlates with sympathetic denervation of the rat myometrium.

Estrogen inhibits the growth and causes the degeneration (pruning) of sympathetic nerves supplying the rat myometrium. Previous cryoculture studies evidenced that substrate-bound signals contribute to diminish the ability of the estrogenized myometrium to support sympathetic nerve growth. Using electron microscopy, here we examined neurite-substrate interactions in myometrial cryocultures, observing that neurites grew associated to collagen fibrils present in the surface of the underlying cryosection. In addition, we assessed quantitatively the effects of estrogen on myometrial collagen organization in situ, using ovariectomized rats treated with estrogen and immature females undergoing puberty. Under low estrogen levels, most collagen fibrils were oriented in parallel to the muscle long axis (83% and 85%, respectively). Following estrogen treatment, 89% of fibrils was oriented perpendicularly to the muscle main axis; while after puberty, 57% of fibrils acquired this orientation. Immunohistochemistry combined with histology revealed that the vast majority of fine sympathetic nerve fibers supplying the myometrium courses within the areas where collagen realignment was observed. Finally, to assess whether depending on their orientation collagen fibrils can promote or inhibit neurite outgrowth, we employed cryocultures, now using as substrate tissue sections of rat-tail tendon. We observed that neurites grew extensively in the direction of the parallel-aligned collagen fibrils in the tendon main axis but were inhibited to grow perpendicularly to this axis. Collectively, these findings support the hypothesis that collagen reorientation may be one of the factors contributing to diminish the neuritogenic capacity of the estrogen-primed myometrial substrate.

Possible role of collagen reorientation during airway remodeling on mucosal folding

Airway remodeling in patients with chronic asthma is characterized by thickening and structural change of airway walls. It has been demonstrated in previous theoretical models that changes in airway thickness can have an important mechanical effect, in particular, on the mucosal folding induced by smooth muscle contraction. In this paper, we examine the effect of collagen fiber orientation change on the mucosal folding. The airway is modeled as a two-layer cylindrical tube, with the submucosa (outer layer) represented as isotropic tissue and subepithelial collagen layer (inner layer) as fiber-reinforced anisotropic tissue. In order to follow the most recent advancement for modeling anisotropic material, we carry out computation under a reduced form of strain energy, as well as a form of strain energy compatible with linear theory in infinitesimal strains. We explore the effects of varying collagen density, strain-stiffening behavior of collagen fibers as well as the layer thickness change on the folding behavior. We demonstrate that collagen orientation change can have a significant impact on mucosal folding, including the critical pressure required to induce folding and the buckling mode. We demonstrate that proper collagen density combined with proper collagen orientation might be a strategy used by organisms to prevent low-mode buckling thus to prevent airway narrowing. Deviation from these balances caused by disease status will favor low-mode buckling. We also find that the two forms of strain energy give very similar results, and therefore, most important features of buckling behavior can still be captured by the reduced form. We conclude that anisotropic property of the subepithelial collagen layer may be an important component in airway remodeling.

Deformation patterns of cracked articular cartilage under compression

Purpose: It is widely accepted that the mechanical environment surrounding cartilage cells (chondrocytes) has a regulatory role on their metabolism. Very early osteoarthritis (OA) is associated with micro-cracks of the cartilage surface that are thought to alter the mechanical environment of chondrocytes, and hence their metabolism. In this study, we assessed the differences of cartilage deformation patterns between intact and cracked cartilage. Methods: Articular cartilage split line patterns of New Zealand white Rabbits were identified using India-ink. Cracks were made at full thickness of cartilage at 90° to the cartilage surface and oriented perpendicular to the split lines. A controlled load of 2MPa was applied, and stress relaxation observed. Cartilage tissues were then fixed in the compressed state and prepared for histology. Samples were mounted either in plastic and stained with Toluidine blue or mounted in paraffin and stained with Picrosirius red for viewing under a polarized light microscope. Results: The split line patterns of patella, femur, and tibia were found to be consistent. Histological analysis revealed that chondrocyte clusters in the radial zone show compression in the vertical direction and realignment of their vertical long axis to an oblique pattern pointing away from the center of the compression site. Under Linear Polarized Light (LPL), extensive collagen fiber reorientation is apparent as they deform in a combination of crimp and bending (Figure 1). Figure 1, Rabbit tibia mounted in plastic, stained with toluidine blue and viewed under LPL. a, Uncompressed tibia. b, Compressed tibia showing the reoriented collagen fibers (fine blue lines). c, Crack location magnified. Under Circular Polarized Light (CPL), the pattern of collagen fiber reorientation that occurred in the cracked vs. the intact samples is different. There is an upside triangle-shaped zone with the crack representing the center. Collagen fibers at the side of the crack are less reoriented compared to those located farther away, indicating the effect of stress release at the crack edge. The sides of the triangle run at an angle similar to that of the oblique running fibers that are normally present in the radial zone of the cartilage, hence this pattern may emphasize the role of these oblique fibers in supporting compressive loads (Figure 2). Figure 2, Rabbit tibia viewed under CPL. a, Intact compressed tibia cartilage. b, Cracked compressed tibia cartilage highlighting the upside triangle pattern. Conclusions: The combination of crimp and bending of the collagen fibres during deformation may have a protective effect and reduce the amount of deformation transmitted to chondrocytes. The presence of a crack completely changed the deformation patterns in the collagen fibers. This is the first time where the micro-structural architecture and deformation of cracked cartilage under compressive loading has been quantified. The difference in collagen reorientation between intact and cracked samples may provide crucial insight into the load distribution in early OA cartilage that contains cracks, and may help identify how cracked cartilage degenerates. Structural changes in surface zone cartilage are among the earliest signs of very early OA and an understanding of these changes in mechanics and cell signaling may allow for altering the time course of OA by either mechanical or pharmacological intervention.

Dense collagen-I matrices enhance pro-tumorigenic estrogen-prolactin crosstalk in MCF-7 and T47D breast cancer cells.

Breast cancers that express estrogen receptor alpha (ERα+) constitute the majority of breast tumors. Estrogen is a major driver of their growth, and targeting ER-mediated signals is a largely successful primary therapeutic strategy. Nonetheless, ERα+ tumors also result in the most breast cancer mortalities. Other factors, including altered characteristics of the extracellular matrix such as density and orientation and consequences for estrogen crosstalk with other hormones such as prolactin (PRL), may contribute to these poor outcomes. Here we employed defined three dimensional low density/compliant and high density/stiff collagen-I matrices to investigate the effects on 17β-estradiol (E2) activity and PRL/E2 interactions in two well-characterized ERα+/PRLR+ luminal breast cancer cell lines in vitro. We demonstrate that matrix density modulated E2-induced transcripts, but did not alter the growth response. However, matrix density was a potent determinant of the behavioral outcomes of PRL/E2 crosstalk. High density/stiff matrices enhanced PRL/E2-induced growth mediated by increased activation of Src family kinases and insensitivity to the estrogen antagonist, 4-hydroxytamoxifen. It also permitted these hormones in combination to drive invasion and modify the alignment of collagen fibers. In contrast, low density/compliant matrices allowed modest if any cooperation between E2 and PRL to growth and did not permit hormone-induced invasion or collagen reorientation. Our studies demonstrate the power of matrix density to determine the outcomes of hormone actions and suggest that stiff matrices are potent collaborators of estrogen and PRL in progression of ERα+ breast cancer. Our evidence for bidirectional interactions between these hormones and the extracellular matrix provides novel insights into the regulation of the microenvironment of ERα+ breast cancer and suggests new therapeutic approaches.