Molecular level detection and localization of mechanical damage in collagen enabled by collagen hybridizing peptides

Softening of elastic and viscoelastic properties is independent of overstretch rate in cerebral arteries

Collagenous soft tissues are frequently injured by supraphysiologic mechanical deformation, leading to measurable changes in both extra-cellular matrix (ECM) structure and mechanical properties. While each of these alterations has been well studied following quasi-static deformation, little is known about the influence of high strain rate. Previous investigations of high-rate ECM alterations found tropocollagen denaturation and fibrillar kinking to be rate dependent. Given these observations of rate dependence in microstructure alterations, the present work evaluated if the rate and magnitude of overstretch affect the baseline viscoelastic properties of porcine middle cerebral arteries (MCAs). Changes in tissue response were assessed using a series of harmonic oscillations before and after sub-failure overstretches across a large range of rates and magnitudes. We used collagen-hybridizing peptide (CHP) to evaluate the role of tropocollagen denaturation in mechanical softening. Experiments show that softening is dependent on overstretch magnitude but is independent of overstretch rate. We also note that softening progresses at the same rate for both equilibrium (quasi-static) and non-equilibrium (high-rate) properties. Finally, results suggest that tropocollagen denaturation is not the source of the observed sub-yield softening behavior. This study expands fundamental knowledge on the form-function relationship of constituents in collagen fibrils and clarifies material behavior following sub-failure overstretch across a range of strain rates.

Correlation of collagen damage and failure mechanics in porcine pia-arachnoid complex

Mild traumatic brain injury (mTBI) accounts for approximately 75% of all TBI cases, and the mechanisms are still poorly understood, in part due to limitations of current diagnostic tools. Yet, there is a critical need to detect the presence of mTBI to mitigate risk of further injury. In this study, we explore the potential of collagen hybridizing peptides (CHPs), which selectively bind to damaged collagen, to detect damage in the pia-arachnoid complex (PAC), a major load-transferring interface during head trauma. To generate damage, porcine PAC samples underwent peel tests. Peak force to failure and CHP fluorescence were measured in three regions of the brain at multiple post-mortem times. The peak force of PAC failure was region-specific, with increasing failure forces moving anterior to posterior (frontal: 20.91 ± 38.77 mN; parietal: 64.72 ± 33.31 mN; occipital: 86.68 ± 43.46 mN) and significantly different between frontal and occipital regions (p = 0.034). CHP fluorescence was significantly different between control and peeled PAC samples in mean pixel intensity (MPI; p = 0.031), median pixel intensity (MedPI; p = 0.009), and percent pixels above a defined threshold (PP; p = 0.014). Each of these CHP fluorescence metrics were significantly and positively correlated with peak force at failure (MPI: p = 0.049; MedPI: p = 0.026; PP: p = 0.002). These data suggest CHP is a viable solution to detecting the presence and severity of damage at the brain-skull interface, and may be a useful tool for quantifying damage in vivo.

Collablots: Quantification of Collagen VI Levels and Its Structural Disorganisation in Cell Cultures From Patients With Collagen VI-Related Dystrophies

AIMS

This study aims to develop a quantitative method for assessing collagen VI expression in cell cultures, which is crucial for the diagnosis and treatment of collagen VI-related dystrophies.

METHODS

We developed a combined in-cell western (ICW) and on-cell western (OCW) assay, which we have called 'collablot', to quantify collagen VI and its organisation in the extracellular matrix of cell cultures from patients and healthy controls. To optimise it, we optimised cell density and the protocols to induce collagen expression in cultures, as well as the cell fixation and permeabilisation methods. This was completed with a thorough selection of collagen antibodies and a collagen-hybridising peptide (CHP). We then used collablots to compare cultures from patients and controls and evaluate therapeutic interventions in the cultures.

RESULTS

Collablots enabled the quantification of collagen VI expression in both control and patient cells, aligning with immunocytochemistry findings and detecting variations in collagen VI expression following treatment of the cultures. Additionally, CHP analysis revealed a marked increase in collagen network disruption in patients compared to the controls.

CONCLUSIONS

The collablot assay represents a suitable method for quantifying collagen VI expression and its organisation in culture and assessing the effect of therapies.

Dynamic hierarchical ligand anisotropy for competing macrophage regulation in vivo

Diverse connective tissues exhibit hierarchical anisotropic structures that intricately regulate homeostasis and tissue functions for dynamic immune response modulation. In this study, remotely manipulable hierarchical nanostructures are tailored to exhibit multi-scale ligand anisotropy. Hierarchical nanostructure construction involves coupling liganded nanoscale isotropic/anisotropic Au (comparable to few integrin molecules-scale) to the surface of microscale isotropic/anisotropic magnetic Fe3O4 (comparable to integrin cluster-scale) and then elastically tethering them to a substrate. Systematic independent tailoring of nanoscale or microscale ligand isotropy versus anisotropy in four different hierarchical nanostructures with constant liganded surface area demonstrates similar levels of integrin molecule bridging and macrophage adhesion on the nanoscale ligand isotropy versus anisotropy. Conversely, the levels of integrin cluster bridging across hierarchical nanostructures and macrophage adhesion are significantly promoted by microscale ligand anisotropy compared with microscale ligand isotropy. Furthermore, microscale ligand anisotropy dominantly activates the host macrophage adhesion and pro-regenerative M2 polarization in vivo over the nanoscale ligand anisotropy, which can be cyclically reversed by substrate-proximate versus substrate-distant magnetic manipulation. This unprecedented scale-specific regulation of cells can be diversified by unlimited tuning of the scale, anisotropy, dimension, shape, and magnetism of hierarchical structures to decipher scale-specific dynamic cell-material interactions to advance immunoengineering strategies.

Functional tendon regeneration is driven by regulatory T cells and IL-33 signaling

Tendon injuries heal by scar, leading to poor function. To date, the role of immune cells remains underexplored. Using a neonatal mouse model of functional tendon healing compared to adult scar-mediated healing, we identified a regenerative immune profile that is associated with type 1 inflammation followed by rapid polarization to type 2, driven by macrophages and regulatory T cells (Treg cells). Single-cell and bulk RNA sequencing also revealed neonatal Treg cells with an immunomodulatory signature distinct from adult. Neonatal Treg cell ablation resulted in a dysregulated immune response, failed tenocyte recruitment, and impaired regeneration. Adoptive transfer further confirmed the unique capacity of neonatal Treg cells to rescue functional regeneration. We showed that neonatal Treg cells mitigate interleukin-33 (IL-33) to enable tenocyte recruitment and structural restoration, and that adult IL-33 deletion improves functional healing. Collectively, these findings demonstrate that Treg cells and IL-33 immune dysfunction are critical components of failed tendon healing and identify potential targets to drive tendon regeneration.

Topography-guided, patterned, customized corneal crosslinking for non-invasive astigmatism correction

OBJECTIVES

To propose and evaluate a novel, non-invasive approach for enduring corneal astigmatism correction based on topography-guided, patterned, customized riboflavin-ultraviolet A corneal collagen crosslinking (CXL).

METHODS

Astigmatism was modelled on both eyes of rabbits. A randomly selected eye of each rabbit was treated by the proposed CXL procedure with another eye as control. The proposed procedure was performed by a self-built intelligent platform through delivering ultraviolet A lattice in a refined and patterned manner, based on pre-operative corneal topography. The long-term effectiveness, stability, and safety were investigated for 180 days, with topographic measurements, anterior segment optical coherence tomography (AS-OCT), and in vivo corneal confocal microscopy (IVCM).

RESULTS

Spatially selective demarcation lines in AS-OCT images and trabecular patterned hyperdense structure with abundant needle-like processes in IVCM images were detected in the CXL eyes, revealing spatially selective crosslinking. Reductions of astigmatic magnitude (in the steep axis: 0.46 ± 0.28 vs. 2.15 ± 0.58 dioptres, P < 0.001) and high order aberration (0.38 ± 0.18 vs. 0.59 ± 0.19, P = 0.009) with increase of visual strehl ratio (0.21 ± 0.06 vs. 0.13 ± 0.03, P < 0.001) were found in the CXL eyes after CXL and maintained for 180 days, compared to inconspicuous changes in the control eyes. No obvious opacity and inflammation were observed in the CXL eyes, and transient loss of endothelial cells in the treated area was recovered in the subsequent visit.

CONCLUSIONS

The proposed novel, non-invasive approach safely fulfilled corneal astigmatism correction with visual quality improvement as well as a decrease in high-order aberration.

Microstructural and Micromechanical Properties of Decellularized Fibrocartilaginous Scaffold

Fibrocartilage decellularized extracellular matrix (dECM) is a promising alternative material for damaged fibrocartilage repair and replacement due to its biomimetic gross morphology and internal microstructure. However, the alterations in the microstructure and micromechanical properties of fibrocartilage after decellularization interfere with the macroscopic functional application of the scaffold. Therefore, this study aims to present an analytical atlas of the microstructure and micromechanics of the fibrocartilaginous dECM scaffold to elucidate the effect of decellularization treatment on the macroscopic function of the scaffold. The fibrocartilage dECM was prepared using the temporomandibular joint (TMJ) disc as the model, and its durability was evaluated under three functional states (physiological, physiological limit, and beyond the limit). The macroscopic function of different fibrocartilage dECM exhibits notable differences, which are attributed to the destruction of the multilevel collagen structure. This process involves unwinding triple-helix tropocollagen molecules, destroying collagen fibril D-periodicity, expanding collagen fiber bundle curling, and loosening of the collagen fiber network. The impairment of multiscale collagen structures degrades the cross-scale mechanical modulus and energy dissipation of dECM from the triple helix molecules to the fibril level to the fiber bundle that extends to the fiber network. This study provides important data for further optimizing decellularized fibrocartilage scaffolds and evaluating their translational potential.

Pregnancy and age differentially affect stiffness, injury susceptibility, and composition of murine uterosacral ligaments

Pelvic organ prolapse is a debilitating condition that diminishes quality of life, and it has been linked to pregnancy and aging. Injury of the uterosacral ligaments (USLs), which provide apical support to the pelvic organs, is a major cause of uterine prolapse. In this study, we examined the effect of pregnancy and age on the apparent elastic modulus, susceptibility to collagen damage, and extracellular matrix (ECM) composition of the murine USL. USLs from mice at three different stages of pregnancy and across two age groups were mechanically tested and evaluated for collagen microdamage. Raman spectroscopy was used to evaluate changes in ECM composition. Our findings reveal that (1) all USLs subjected to mechanical stretch sustained collagen microdamage, (2) both pregnancy and age significantly affected USL stiffness and injury susceptibility, and (3) pregnancy, but not age, altered ECM composition. Overall, this work represents a major step toward understanding the role of tissue microstructure and mechanical function in USL injury, which should guide novel ECM-targeted treatment and prevention strategies for uterine prolapse.

Strategic enhancement of collagen detection using lanthanide-functionalized collagen targeted peptides

Monitoring collagen denaturation is crucial for diagnosing collagen-related diseases such as tumors and fibrosis. Herein, we have developed specific probes to detect denatured collagen (d-Col) and collagen I (Col I), utilizing peptide probes with sequences (GOP)10 and HVWMQAP, targeting at d-Col and Col I, respectively. These peptides were conjugated with 1,10-phenanthroline-5-carboxylic Acid (Phen), forming Phen-Ahx-(GOP)10 and Phen-Ahx-HVWMQAP. Phen acts as both an antenna sensitizer and a chelator, coordinating with Terbium (III) and Europium (III) ions via its nitrogen atom, facilitating fluorescent emission in green and red, respectively. The investigation demonstrated that Tb3+ interacts with three (GOP)10 peptide units through Phen, while Eu3+ connects with four units of Ahx-HVWMQAP peptides. Additionally, it is important to note that the structure of the peptides remains unchanged after chelating with the lanthanide ions, maintaining their integrity throughout the process. These probes have effectively demonstrated their ability to bind to specific collagen types with selectivity, enabling accurate identification of their presence. The excellent binding of these probes is due to the branched structure of the formed lanthanide-peptide complexes. A dose-dependent linear association was observed in the binding of Eu-[Phen-Ahx-HVWMQAP]4 to Col I, with concentration levels ranging from 0.5 to 100 μM and a minimal detectable concentration of 0.113 μM. We anticipate that our developed probes will improve understanding of collagen remodeling and provide opportunities for the diagnosis of collagen-associated diseases.

Fluorescein-based SynNotch adaptors for regulating gene expression responses to diverse extracellular and matrix-based cues

Synthetic Notch (SynNotch) receptors function like natural Notch proteins and can be used to install customized sense-and-respond capabilities into mammalian cells. Here, we introduce an adaptor-based strategy for regulating SynNotch activity via fluorescein isomers and analogs. Using an optimized fluorescein-binding SynNotch receptor, we describe ways to chemically control SynNotch signaling, including an approach based on a bio-orthogonal chemical ligation and a spatially controllable strategy via the photo-patterned uncaging of an o-nitrobenzyl-caged fluorescein conjugate. We further show that fluorescein-conjugated extracellular matrix (ECM)-binding peptides can be used to regulate SynNotch activity depending on the folding state of collagen-based ECM networks. To demonstrate the utility of these tools, we apply them to activate dose-dependent gene expression responses and to induce myogenic-like phenotypes in multipotent fibroblasts with spatiotemporal and microenvironmental control. Overall, we introduce an optimized fluorescein-binding SynNotch as a versatile tool for regulating transcriptional responses to ligands based on the clinically-approved fluorescein dye.

Fibre type differences in the organisation of mononuclear cells and myonuclei at the tips of human myofibres

The myotendinous junction (MTJ) is a weak link in the musculoskeletal system. Here, we isolated the tips of single myofibres from healthy (non-injured) human hamstring muscles for confocal microscopy (n=6) and undertook RNAscope in situ hybridisation (n=6) to gain insight into the profiles of cells and myonuclei in this region, in a fibre type manner. A marked presence of mononuclear cells was observed coating the myofibre tips (confirmed by serial block face scanning electron microscopy and cryosection immunofluorescence), with higher numbers for type I (median 29; range 16-63) than type II (16; 9-23) myofibres (P<0.05). The number of these cells expressing COL22A1 was comparable between fibre types. Myonuclear number and density gradually increased from the myofibre proper towards the tip for both fibre types (P<0.05). COL22A1 was expressed by similar proportions of myonuclei in type I (median 26%; range 13-56) and type II (19%; 3-67) myofibre tips. 70% of the COL22A1-positive nuclei in the MTJ region were myonuclei, and the remaining 30% were MTJ cells. This insight refines our fundamental understanding of the human MTJ at the cell and structural levels.

Modeling Fatigue Failure of Cartilage and Fibrous Biological Tissues Using Constrained Reactive Mixture Theory

Fatigue failure in biological soft tissues plays a critical role in the etiology of chronic soft tissue injuries and diseases such as osteoarthritis (OA). Understanding failure mechanisms is hindered by the decades-long timescales over which damage takes place. Analyzing the factors contributing to fatigue failure requires the help of validated computational models developed for soft tissues. This study presents a framework for fatigue failure of fibrous biological tissues based on reaction kinetics, where the composition of intact and fatigued material regions can evolve via degradation and breakage over time, in response to energy-based fatigue and damage criteria. Using reactive constrained mixture theory, material region mass fractions are governed by the axiom of mass balance. Progression of fatigue is controlled by an energy-based reaction rate, with user-selected probability functions defining the damage propensity of intact and fatigued material regions. Verification of this reactive theory, which is implemented in the open-source FEBio finite element software, is provided in this study. Validation is also demonstrated against experimental data, showing that predicted damage can be linked to results from biochemical assays. The framework is also applied to study fatigue failure during frictional contact of cartilage. Simulating previous experiments suggests that frictional effects slightly increase fatigue progression, but the main driver is cyclic compressive contact loading. This study demonstrated the ability of theoretical models to complement and extend experimental findings, advancing our understanding of the time progression of fatigue in biological tissues.

Peptides as Targeting Agents and Therapeutics: A Brief Overview

The controllability and specificity of peptides make them ideal for targeting therapeutic delivery systems and as therapeutic agents that interfere with the essential functions of pathogens and tumors. Peptides can also mimic natural protein structures or parts thereof, agonize receptors, and be conjugated to other molecules that will self-assemble. In this short Review, we discuss research from the last ten years into peptide use in three arenas: the treatment of cancer, the treatment of pathogens, and the targeting of specific organs and organelles. These studies demonstrate the successful application of targeting and therapeutic peptides in vitro and in vivo and show the promising range of applications peptides can have going forward.

Hidden length lets collagen buffer mechanical and chemical stress

Collagen, the most abundant protein in the human body, must withstand high mechanical loads due to its structural role in tendons, skin, bones, and other connective tissue. It was recently found that tensed collagen creates mechanoradicals by homolytic bond scission. We here employ scale-bridging simulations to determine the influence of collagen's mesoscale fibril structure on molecular breakages, combining atomistic molecular dynamics simulations with a newly developed mesoscopic ultra-coarse-grained description of a collagen fibril. Our simulations identify a conserved structural feature, a length difference of the two helices between pairs of crosslinks, to play a critical role. The release of the extra hidden length enables collagen to buffer mechanical stress. At the same time, this topology funnels ruptures such that the potentially harmful mechanoradicals are readily stabilized, buffering the arising oxidative stress. Our results suggest collagen's hidden length to exploit a sweet spot in the trade-off between breakage specificity and strength.

Aging by autodigestion

The mechanism that triggers the progressive dysregulation of cell functions, inflammation, and breakdown of tissues during aging is currently unknown. We propose here a previously unknown mechanism due to tissue autodigestion by the digestive enzymes. After synthesis in the pancreas, these powerful enzymes are activated and transported inside the lumen of the small intestine to which they are compartmentalized by the mucin/epithelial barrier. We hypothesize that this barrier leaks active digestive enzymes (e.g. during meals) and leads to their accumulation in tissues outside the gastrointestinal tract. Using immune-histochemistry we provide evidence in young (4 months) and old (24 months) rats for significant accumulation of pancreatic trypsin, elastase, lipase, and amylase in peripheral organs, including liver, lung, heart, kidney, brain, and skin. The mucin layer density on the small intestine barrier is attenuated in the old and trypsin leaks across the tip region of intestinal villi with depleted mucin. The accumulation of digestive enzymes is accompanied in the same tissues of the old by damage to collagen, as detected with collagen fragment hybridizing peptides. We provide evidence that the hyperglycemia in the old is accompanied by proteolytic cleavage of the extracellular domain of the insulin receptor. Blockade of pancreatic trypsin in the old by a two-week oral treatment with a serine protease inhibitor (tranexamic acid) serves to significantly reduce trypsin accumulation in organs outside the intestine, collagen damage, as well as hyperglycemia and insulin receptor cleavage. These results support the hypothesis that the breakdown of tissues in aging is due to autodigestion and a side-effect of the fundamental requirement for digestion.

Collagen denaturation in post-run Achilles tendons and Achilles tendinopathy: In vivo mechanophysiology and magnetic resonance imaging

Achilles tendinopathy is often attributed to overuse, but its pathophysiology remains poorly understood. Disruption to the molecular structure of collagen is fundamental for the onset and progression of tendinopathy but has mostly been investigated in vitro. Here, we interrogated the in vivo molecular structure changes of collagen in rat Achilles tendons following treadmill running. Unexpectedly, the tendons' collagen molecules were not mechanically unfolded by running but denatured through proteolysis during physiological post-run remodeling. We further revealed that running induces inflammatory gene expressions in Achilles tendons and that long-term running causes prolonged, elevated collagen degradation, leading to the accumulation of denatured collagen and tendinopathy development. For applications, we demonstrated magnetic resonance imaging of collagenase-induced Achilles tendon injury in vivo using a denatured collagen targeting contrast agent. Our findings may help close the knowledge gaps in the mechanobiology and pathogenesis of Achilles tendinopathy and initiate new strategies for its imaging-based diagnosis.

Collagen integrity of the annulus fibrosus in degenerative disc disease individuals quantified with collagen hybridizing peptide

Introduction

Degenerative disc disease (DDD) is accompanied by structural changes in the intervertebral discs (IVD). Extra-cellular matrix degradation of the annulus fibrosus (AF) has been linked with degeneration of the IVD. Collagen is a vital component of the IVD. Collagen hybridizing peptide (CHP) is an engineered protein that binds to degraded collagen, which we used to quantify collagen damage in AF. This method was used to compare AF samples obtained from donors with no DDD to AF samples from patients undergoing surgery for symptomatic DDD.

Methods

Fresh AF tissue was embedded in an optimal cutting temperature compound and cryosectioned at a thickness of 8 μm. Hematoxylin and Eosin staining was performed on sections for general histomorphological assessment. Serial sections were stained with Cy3-conjugated CHP and the mean fluorescence intensity and areal fraction of Cy3-positive staining were averaged for three regions of interest (ROI) on each CHP-stained section.

Results

Increases in mean fluorescence intensity (p = 0.0004) and percentage of positively stained area (p = 0.00008) with CHP were detected in DDD samples compared to the non-DDD samples. Significant correlations were observed between mean fluorescence intensity and percentage of positively stained area for both non-DDD (R = 0.98, p = 5E-8) and DDD (R = 0.79, p = 0.0012) samples. No significant differences were detected between sex and the lumbar disc level subgroups of the non-DDD and DDD groups. Only tissue pathology (non-DDD versus DDD) influenced the measured parameters. No three-way interactions between tissue pathology, sex, and lumbar disc level were observed.

Discussion and Conclusions

These findings suggest that AF collagen degradation is greater in DDD samples compared to non-DDD samples, as evidenced by the increased CHP staining. Strong positive correlations between the two measured parameters suggest that when collagen degradation occurs, it is detected by this technique and is widespread throughout the tissue. This study provides new insights into the structural alterations associated with collagen degradation in the AF that occur during DDD.

In vivo monitoring of active subretinal fibrosis in mice using collagen hybridizing peptides

Subretinal fibrosis is associated with worse visual outcomes in patients with neovascular age-related macular degeneration. As there is a lack of optimal biomarkers and no method that directly detects collagen in the back of the eye, novel tools that monitor fibrosis-related changes in neovascular age-related macular degeneration are needed. Here, using two mouse models (the laser-induced choroidal neovascularization model, and the JR5558 mouse presenting with spontaneous subretinal neovascularization with fibrosis), we imaged active fibrotic lesions using fluorescently labeled collagen hybridizing peptides (CHPs), short peptides that bind to single α-chain collagen structures during collagen remodeling. JR5558 retinal pigment epithelium/choroid flat mounts showed CHP co-staining with fibrosis and epithelial mesenchymal transition-related markers; additionally, CHP histopathology staining correlated with in vivo CHP imaging. After laser-induced choroidal neovascularization, in vivo CHP binding correlated with laser intensity, histopathology CHP and fibronectin staining. Laser-induced choroidal neovascularization showed decreased CHP intensity over time in healing/regressing versus active scars in vivo, whereas increased CHP binding correlated with elevated fibrosis in JR5558 mouse eyes with age. In bispecific angiopoietin 2/vascular endothelial growth factor antibody-treated JR5558 mice, CHPs detected significantly decreased collagen remodeling versus immunoglobulin G control. These results demonstrate the first use of CHPs to directly image remodeling collagen in the eye and as a potential clinical optical biomarker of active subretinal fibrosis associated with ocular neovascularization.

Type VI Collagen Deficiency Causes Enhanced Periodontal Tissue Destruction

The periodontal ligament (PDL) is a fibrillar connective tissue that lies between the alveolar bone and the tooth and is composed of highly specialized extracellular matrix (ECM) molecules and a heterogeneous population of cells that are responsible for collagen formation, immune response, bone formation, and chewing force sensation. Type VI collagen (COL6), a widely distributed ECM molecule, plays a critical role in the structural integrity and mechanical properties of various tissues including muscle, tendon, bone, cartilage, and skin. However, its role in the PDL remains largely unknown. Our study shows that deficiency of COL6 impairs PDL fibrillogenesis and exacerbates tissue destruction in ligature-induced periodontitis (LIP). We found that COL6-deficient mice exhibited increased bone loss and degraded PDL in LIP and that fibroblasts expressing high levels of Col6α2 are pivotal in ECM organization and cell-ECM interactions. Moreover, COL6 deficiency in the PDL led to an increased number of fibroblasts geared toward the inflammatory response. We also observed that cultured COL6-deficient fibroblasts from the PDL exhibited decreased expression of genes related to collagen fiber turnover and ECM organization as well as migration and proliferation. Our findings suggest that COL6 plays a crucial role in the PDL, influencing fibroblast function in fibrillogenesis and affecting the immune response in periodontitis. These insights advance our understanding of the molecular mechanisms underlying PDL maturation and periodontal disease.

Age-dependent changes in collagen crosslinks reduce the mechanical toughness of human meniscus

The mechanical resilience of the knee meniscus is provided by a group of structural proteins in the extracellular matrix. Aging can alter the quantity and molecular structure of these proteins making the meniscus more susceptible to debilitating tears. In this study, we determined the effect of aging on the quantity of structural proteins and collagen crosslinks in human lateral meniscus, and examined whether the quantity of these molecules was predictive of tensile toughness (area under the stress-strain curve). Two age groups were tested: a young group under 40 and an older group over 65 years old. Using mass spectrometry, we quantified the abundance of proteins and collagen crosslinks in meniscal tissue that was adjacent to the dumbbell-shaped specimens used to measure uniaxial tensile toughness parallel or perpendicular to the circumferential fiber orientation. We found that the enzymatic collagen crosslink deoxypyridinoline had a significant positive correlation with toughness, and reductions in the quantity of this crosslink with aging were associated with a loss of toughness in the ground substance and fibers. The non-enzymatic collagen crosslink carboxymethyl-lysine increased in quantity with aging, and these increases corresponded to reductions in ground substance toughness. For the collagenous (Types I, II, IV, VI, VIII) and non-collagenous structural proteins (elastin, decorin, biglycan, prolargin) analyzed in this study, only the quantity of collagen VIII was predictive of toughness. This study provides valuable insights on the structure-function relationships of the human meniscus, and how aging causes structural adaptations that weaken the tissue's mechanical integrity.

From Collagen Mimetics to Collagen Hybridization and Back

Facilitated by the unique triple-helical protein structure, fibrous collagens, the principal proteins in animals, demonstrate a dual function of serving as building blocks for tissue scaffolds and as a bioactive material capable of swift renewal in response to environmental changes. While studies of triple-helical collagen mimetic peptides (CMPs) have been instrumental in understanding the molecular forces responsible for the folding and assembly of triple helices, as well as identifying bioactive regions of fibrous collagen molecules, single-strand CMPs that can specifically target and hybridize to denatured collagens (i.e., collagen hybridizing peptides, CHPs) have proven useful in identifying the remodeling activity of collagen-rich tissues related to development, homeostasis, and pathology. Efforts to improve the utility of CHPs have resulted in the development of new skeletal structures, such as dimeric and cyclic CHPs, as well as the incorporation of artificial amino acids, including fluorinated proline and N-substituted glycines (peptoid residues). In particular, dimeric CHPs were used to capture collagen fragments from biological fluid for biomarker study, and the introduction of peptoid-based collagen mimetics has sparked renewed interest in peptidomimetic research because peptoids enable a stable triple-helical structure and the presentation of an extensive array of side chain structures offering a versatile platform for the development of new collagen mimetics. This Account will cover the evolution of our research from CMPs as biomaterials to ongoing efforts in developing triple-helical peptides with practical theranostic potential in targeting denatured and damaged collagens. Our early efforts in functionalizing natural collagen scaffolds via noncovalent modifications led to the discovery of an entirely new use of CMPs. This discovery resulted in the development of CHPs that are now used by many different laboratories for the investigation of pathologies associated with changes in the structures of extracellular matrices including fibrosis, cancer, and mechanical damage to collagen-rich, load-bearing tissues. Here, we delve into the essential design features of CHPs contributing to their collagen binding properties and practical usage and explore the necessity for further mechanistic understanding of not only the binding processes (e.g., binding domain and stoichiometry of the hybridized complex) but also the biology of collagen degradation, from proteolytic digestion of fibrils to cellular processing of collagen fragments. We also discuss the strengths and weaknesses of peptoid-based triple-helical peptides as applied to collagen hybridization touching on thermodynamic and kinetic aspects of triple-helical folding. Finally, we highlight current limitations and future directions in the use of peptoid building blocks to develop bioactive collagen mimetics as new functional biomaterials.

Fluorescein-Based SynNotch Adaptors for Regulating Gene Expression Responses to Diverse Extracellular Cues

We introduce an adaptor-based strategy for regulating fluorescein-binding synthetic Notch (SynNotch) receptors using ligands based on conjugates of fluorescein isomers and analogs. To develop a versatile system, we evaluated the surface expression and activities of multiple constructs containing distinct extracellular fluorescein-binding domains. Using an optimized receptor, we devised ways to regulate signaling via fluorescein-based chemical transformations, including an approach based on a bio-orthogonal chemical ligation and a spatially controllable strategy via the photo-patterned uncaging of an o -nitrobenzyl-caged fluorescein conjugate. We further demonstrate that fluorescein-conjugated extracellular matrix (ECM)-binding peptides can regulate SynNotch activity depending on the folding state of collagen-based ECM networks. Treatment with these conjugates enabled cells to distinguish between folded versus denatured collagen proteins and enact dose-dependent gene expression responses depending on the nature of the signaling adaptors presented. To demonstrate the utility of these tools, we applied them to control the myogenic conversion of fibroblasts into myocytes with spatial and temporal precision and in response to denatured collagen-I, a biomarker of multiple pathological states. Overall, we introduce an optimized fluorescein-binding SynNotch as a versatile tool for regulating transcriptional responses to extracellular ligands based on the widely used and clinically-approved fluorescein dye.

Controlled Release of Drugs from Extracellular Matrix-Derived Peptide-Based Nanovesicles through Tailored Noncovalent Interactions

Elastin-collagen nanovesicles (ECnV) have emerged as a promising platform for drug delivery due to their tunable physicochemical properties and biocompatibility. The potential of nine distinct ECnVs to serve as drug-delivery vehicles was investigated in this study, and it was demonstrated that various small-molecule cargo (e.g., dexamethasone, methotrexate, doxorubicin) can be encapsulated in and released from a set of ECnVs, with extents of loading and rates of release dictated by the composition of the elastin domain of the ECnV and the type of cargo. Elastin-like peptides (ELPs) and collagen-like peptides (CLPs) of various compositions were produced; the secondary structure of the corresponding peptides was determined using CD, and the morphology and average hydrodynamic diameter (∼100 nm) of the ECnVs were determined using TEM and DLS. It was observed that hydrophobic drugs exhibited slower release kinetics than hydrophilic drugs, but higher drug loading was achieved for the more hydrophilic Dox. The collagen-binding ability of the ECnVs was demonstrated through a 2D collagen-binding assay, suggesting the potential for longer retention times in collagen-enriched tissues or matrices. Sustained release of drugs for up to 7 days was observed and, taken together with the collagen-binding data, demonstrates the potential of this set of ECnVs as a versatile drug delivery vehicle for longer-term drug release of a variety of cargo. This study provides important insights into the drug delivery potential of ECnVs and offers useful information for future development of ECnV-based drug delivery systems for the treatment of various diseases.

An interferometric-based tensile tester to resolve damage events within reconstituted multi-filaments collagen bundles

Understanding how mechanical damage propagates in load-bearing tissues such as skin, tendons and ligaments, is key to developing regenerative medicine solutions for when these tissues fail. For collagenous tissues in particular, damage is typically assessed after mechanical testing using a broad range of microscopy techniques because standard tensile testing systems do not have the time and force sensitivity to resolve mechanical damage events. Here we introduce an interferometric detection scheme to measure the displacement of a cantilever with a resolution of 0.03% of full scale at a sampling rate of 5000 samples/s. The system is validated using collagen fibers engineered to mimic mammalian tendons. The system can detect sudden decrease in force due to slippage between collagen filaments, one to five microns in diameter, within a fiber in air. It can also detect yield events associated with local collagen unfolding or sliding within collagen fibrils within a fiber in liquid. This is opening the road to the sub-failure study of damage propagation within a broad range of hierarchical biomaterials.

Multimodal experimental studies of the passive mechanical behavior of human aortas: Current approaches and future directions

Cardiovascular diseases are the leading cause of death worldwide and include, among others, critical conditions of the aortic wall. Importantly, such critical conditions require effective diagnosis and treatment, which are not yet accurate enough. However, they could be significantly strengthened with predictive material models of the aortic wall. In particular, such predictive models could support surgical decisions, preoperative planning, and estimation of postoperative tissue remodeling. However, developing a predictive model requires experimental data showing both structural parameters and mechanical behavior. Such experimental data can be obtained using multimodal experiments. This review therefore discusses the current approaches to multimodal experiments. Importantly, the strength of the aortic wall is determined primarily by its passive components, i.e., mainly collagen, elastin, and proteoglycans. Therefore, this review focuses on multimodal experiments that relate the passive mechanical behavior of the human aortic wall to the structure and organization of its passive components. In particular, the multimodal experiments are classified according to the expected results. Multiple examples are provided for each experimental class and summarized with highlighted advantages and disadvantages of the method. Finally, future directions of multimodal experiments are envisioned and evaluated. STATEMENT OF SIGNIFICANCE: Multimodal experiments are innovative approaches that have gained interest very quickly, but also recently. This review presents therefore a first clear summary of groundbreaking research in the field of multimodal experiments. The benefits and limitations of various types of multimodal experiments are thoroughly discussed, and a comprehensive overview of possible results is provided. Although this review focuses on multimodal experiments performed on human aortic tissues, the methods used and described are not limited to human aortic tissues but can be extended to other soft materials.

The electron beam irradiation of collagen in the dry and gel states: The effect of the dose and water content from the primary to the quaternary levels

The aim of our study was to describe the impact of collagen in the gel and dry state to various doses of electron beam radiation (1, 10 and 25 kGy) which are using for food processing and sterilization. The changes in the chemical compositions (water, amino acids, lipids, glycosaminoglycans) were analyzed and the changes in the structure (triple-helix or β-sheet, the integrity of the collagen) were assessed. Subsequently, the impact of the applied doses on the mechanical properties, stability in the enzymatic environment, swelling and morphology were determined. The irradiated gels evinced enhanced degrees of cross-linking with only partial degradation. Nevertheless, an increase was observed in their stability manifested via a higher degree of resistance to the enzymatic environment, a reduction in swelling and, in terms of the mechanical behaviour, an approximation to the non-linear behavior of native tissues. In contrast, irradiation in the dry state exerted a somewhat negative impact on the observed properties and was manifested mainly via the scission of the collagen molecule and via a lower degree of stability in the aqueous and enzymatic environments. Neither the chemical composition nor the morphology was affected by irradiation.

Phenolated alginate hydrogel induced osteogenic properties of mesenchymal stem cells via Wnt signaling pathway

Osteogenic properties of phenolated alginate (1.2 %) hydrogel containing collagen (0.5 %)/nano-hydroxyapatite (1 %) were studied on human mesenchymal stem cells in vitro. The phenolation rate and physical properties of the hydrogel were assessed using nuclear magnetic resonance (NMR), Fourier-transform infrared spectroscopy (FTIR), Scanning electron microscope (SEM), swelling ratio, gelation time, mechanical assay, and degradation rate. The viability of encapsulated cells was monitored on days 7, 14, and 21 using an MTT assay. Osteoblast differentiation was studied using western blotting, and real-time PCR. Using PCR array analysis, the role of the Wnt signaling pathway was also investigated. Data showed that the combination of alginate/collagen/nanohydroxyapatite yielded proper mechanical features. The addition of nanohydroxyapatite, and collagen reduced degradation, swelling rate coincided with increased stiffness. Elasticity and pore size were also diminished. NMR and FTIR revealed suitable incorporation of collagen and nanohydroxyapatite in the structure of alginate. Real-time PCR analysis and western blotting indicated the expression of osteoblast-related genes such as Runx2 and osteocalcin. PCR array revealed the induction of numerous genes related to Wnt signaling pathways during the maturation of human stem cells toward osteoblast-like cells. In vivo data indicated that transplantation of phenolated alginate/collagen/nanohydroxyapatite hydrogel led to enhanced de novo bone formation in rats with critical-sized calvarial defects. Phenolated alginate hydrogel can promote the osteogenic capacity of human amniotic membrane mesenchymal stem cells in the presence of nanohydroxyapatite and collagen via engaging the Wnt signaling pathway.

Age-related changes in dermal collagen physical properties in human skin

Collagen is the major structural protein in the skin. Fragmentation and disorganization of the collagen fibrils are the hallmarks of the aged human skin dermis. These age-related alterations of collagen fibrils impair skin structural integrity and make the tissue microenvironment more prone to skin disorders. As the biological function of collagen lies predominantly in its physical properties, we applied atomic force microscopy (AFM) and nanoindentation to evaluate the physical properties (surface roughness, stiffness, and hardness) of dermal collagen in young (25±5 years, N = 6) and aged (75±6 years, N = 6) healthy sun-protected hip skin. We observed that in the aged dermis, the surface of collagen fibrils was rougher, and fiber bundles were stiffer and harder, compared to young dermal collagen. Mechanistically, the age-related elevation of matrix metalloproteinase-1 (MMP-1) and advanced glycation end products (AGEs) are responsible for rougher and stiffer/harder dermal collagen, respectively. Analyzing the physical properties of dermal collagen as a function of age revealed that alterations of the physical properties of collagen fibrils changed with age (22-89 years, N = 18). We also observed that the reticular dermis is rougher and mechanically stiffer and harder compared to the papillary dermis in human skin. These data extend the current understanding of collagen beyond biological entities to include biophysical properties.

Targeting damaged collagen for intra-articular delivery of therapeutics using collagen hybridizing peptides

The objective of this study was to investigate the potential of collagen hybridizing peptides (CHPs), which bind to denatured collagen, to extend the retention time of near-infrared fluorophores (NIRF) following intra-articular (IA) injection in rat knee joints. CHPs were synthesized with a NIRF conjugated to the N-terminus. Male Sprague-Dawley rats were assigned to one of four experimental groups: healthy, CHP; osteoarthritis (OA), CHP; healthy, scrambled-sequence CHP (sCHP), which has no collagen binding affinity; or OA, sCHP. Animals in the OA groups received an IA injection of monosodium iodoacetate to induce OA. All animals then received the corresponding CHP injection. Animals were imaged repeatedly over 2 weeks using an in vivo fluorescence imaging system. Joint components were isolated and imaged to determine CHP binding distribution. Safranin-O and Fast Green histological staining was performed to confirm the development of OA. CHPs were found to be retained within the joint following IA injection in both healthy and OA animals for the full study period. In contrast, sCHP signal was negligible by 24-48 h. CHP signal was significantly greater (p < 0.05) in OA joints when compared to healthy joints. At the 2-week end point, multiple joint components retained CHPs, including cartilage, meniscus, and synovium. CHPs dramatically extended the retention time of NIRFs following IA injection in healthy and OA knee joints by binding to multiple collagenous tissues in the joint. These results support the pursuit of further research to develop CHP based therapeutics for IA treatment of OA.

Histological and immunohistochemical guide to tendon tissue

Tendons are unique dense connective tissues with discrete zones having specific structure and function. They are juxtaposed with other tissues (e.g., bone, muscle, and fat) with different compositional, structural, and mechanical properties. Additionally, tendon properties change drastically with growth and development, disease, aging, and injury. Consequently, there are unique challenges to performing high quality histological assessment of this tissue. To address this need, histological assessment was one of the breakout session topics at the 2022 Orthopaedic Research Society (ORS) Tendon Conference hosted at the University of Pennsylvania. The purpose of the breakout session was to discuss needs from members of the ORS Tendon Section related to histological procedures, data presentation, knowledge dissemination, and guidelines for future work. Therefore, this review provides a brief overview of the outcomes of this discussion and provides a set of guidelines, based on the perspectives from our laboratories, for histological assessment to assist researchers in their quest to utilize these techniques to enhance the outcomes and interpretations of their studies.

Pan-tissue scaling of stiffness versus fibrillar collagen reflects contractility-driven strain that inhibits fibril degradation

Polymer network properties such as stiffness often exhibit characteristic power laws in polymer density and other parameters. However, it remains unclear whether diverse animal tissues, composed of many distinct polymers, exhibit such scaling. Here, we examined many diverse tissues from adult mouse and embryonic chick to determine if stiffness ( E tissue ) follows a power law in relation to the most abundant animal protein, Collagen-I, even with molecular perturbations. We quantified fibrillar collagen in intact tissue by second harmonic generation (SHG) imaging and from tissue extracts by mass spectrometry (MS), and collagenase-mediated decreases were also tracked. Pan-tissue power laws for tissue stiffness versus Collagen-I levels measured by SHG or MS exhibit sub-linear scaling that aligns with results from cellularized gels of Collagen-I but not acellular gels. Inhibition of cellular myosin-II based contraction fits the scaling, and combination with inhibitors of matrix metalloproteinases (MMPs) show collagenase activity is strain - not stress- suppressed in tissues, consistent with past studies of gels and fibrils. Beating embryonic hearts and tendons, which differ in both collagen levels and stiffness by >1000-fold, similarly suppressed collagenases at physiological strains of ∼5%, with fiber-orientation regulating degradation. Scaling of E tissue based on 'use-it-or-lose-it' kinetics provides insight into scaling of organ size, microgravity effects, and regeneration processes while suggesting contractility-driven therapeutics.

A method for defining tissue injury criteria reveals that ligament deformation thresholds are multimodal

Soft tissue injuries (such as ligament, tendon, and meniscus tears) are the result of extracellular matrix damage from excessive tissue stretching. Deformation thresholds for soft tissues, however, remain largely unknown due to a lack of methods that can measure and compare the spatially heterogeneous damage and deformation that occurs in these materials. Here, we propose a full-field method for defining tissue injury criteria: multimodal strain limits for biological tissues analogous to yield criteria that exist for crystalline materials. Specifically, we developed a method for defining strain thresholds for mechanically-driven fibrillar collagen denaturation in soft tissues, using regional multimodal deformation and damage data. We established this new method using the murine medial collateral ligament (MCL) as our model tissue. Our findings revealed that multiple modes of deformation contribute to collagen denaturation in the murine MCL, contrary to the common assumption that collagen damage is driven only by strain in the direction of fibers. Remarkably, hydrostatic strain (computed here with an assumption of plane strain) was the best predictor of mechanically-driven collagen denaturation in ligament tissue, suggesting crosslink-mediated stress transfer plays a role in molecular damage accumulation. This work demonstrates that collagen denaturation can be driven by multiple modes of deformation and provides a method for defining deformation thresholds, or injury criteria, from spatially heterogeneous data. STATEMENT OF SIGNIFICANCE: Understanding the mechanics of soft tissue injuries is crucial for the development of new technology for injury detection, prevention, and treatment.  Yet, tissue-level deformation thresholds for injury are unknown, due to a lack of methods that combine full-field measurements of multimodal deformation and damage in mechanically loaded soft tissues. Here, we propose a method for defining tissue injury criteria: multimodal strain thresholds for biological tissues. Our findings reveal that multiple modes of deformation contribute to collagen denaturation, contrary to the common assumption that collagen damage is driven by strain in the fiber direction alone. The method will inform the development of new mechanics-based diagnostic imaging, improve computational modeling of injury, and be employed to study the role of tissue composition in injury susceptibility.

Cell mediated ECM-degradation as an emerging tool for anti-fibrotic strategy

Investigation into the role of cells with respect to extracellular matrix (ECM) remodeling is still in its infancy. Particularly, ECM degradation is an indispensable process during the recovery from fibrosis. Cells with ECM degradation ability due to the secretion of various matrix metalloproteinases (MMPs) have emerged as novel contributors to the treatment of fibrotic diseases. In this review, we focus on the ECM degradation ability of cells associated with the repertoire of MMPs that facilitate the attenuation of fibrosis through the inhibition of ECM deposition. Besides, innovative approaches to engineering and characterizing cells with degradation ability, as well as elucidating the mechanism of the ECM degradation, are also illustrated. Studies conducted to date on the use of cell-based degradation for therapeutic purposes to combat fibrosis are summarized. Finally, we discuss the therapeutic potential of cells with high degradation ability, hoping to bridge the gap between benchside research and bedside applications in treating fibrotic diseases.

Characterization of the L4/L5 rat facet capsular ligament macromechanical and microstructural responses to tensile failure loading

Low back pain is a prevalent condition that affects the global population. The lumbar facet capsular ligament is a source of pain since the collagenous tissue of the ligament is innervated with sensory neurons that deform with the capsule's stretch. Regional differences in the microstructural and macrostructural anatomy of the spinal facets affect its capsule's mechanical behavior. Although there are many studies of the cervical facet in human and rodent models, the lumbar capsular ligament's multiscale behavior is less well-defined. This study characterizes the macroscale and fiber-scale changes of the rat lumbar facet capsule during tensile failure loading. An integrated polarized light imaging setup captured local fiber alignment during 0.08 mm/s distraction of 7 lumbar facets. Force, displacement, strain, and circular variance were measured at several points along the failure curve: the first instance when the local collagen fiber network realigns differentially (anomalous realignment), yield, the first peak in force corresponding to the capsule's first failure, and peak force, defined as ultimate rupture. Those outcomes were compared across events. While each of force, displacement, and average maximum principal strain increased with applied tension, so did the circular variance of the collagen, suggesting that the fibers were becoming more disorganized. From the fiber alignment maps collected at each mechanical event, the number of anomalous realignment events were counted and found to increase dramatically with loading. The increased collagen disorganization and increasing regions of such disorganization in the facet capsule during loading can provide insights about how loading to the ligament afferent nerves may be activated and thereby produce pain.

Soft, strong, tough, and durable bio-hydrogels via maximizing elastic entropy

Load-bearing soft tissues are soft but strong, strong yet tough. These properties can only be replicated in synthetic hydrogels, which do not have the biocomplexity required by many biomedical applications. By contrast, natural hydrogels, although retaining the native complexity, are weak and fragile. Here we present a thermomechanical casting method to achieve the mechanical capabilities of synthetic materials in biopolymer hydrogels. The thermomechanical cast and chemically crosslinked biopolymer chains form a short-range disordered but long-range ordered structure in water. Upon stretch, the disordered structure transforms to a hierarchically ordered structure. This disorder-order transformation resembles the synergy of the disordered elastin and ordered collagen in load-bearing soft tissues. As entropy drives a reverse order-disorder transformation, the hydrogels can resist repeated cycles of loads without deterioration in mechanical properties. Gelatin hydrogels produced by this method combine tissue-like tunable mechanical properties that outperform the gelatin prepared by synthetic approaches, and in vivo biocomplexity beyond current natural systems. Unlike polymer engineering approaches, which rely on specific crosslinks provided by special polymers, this strategy utilizes the entropy of swollen chains and is generalizable to many other biopolymers. It could thus significantly accelerate translational success of biomaterials.

Correlation between Mechanical Properties and Collagen Degeneration in Fibrous Tissue

Fibrosis is a disease that causes abnormal accumulation of collagen and other extracellular matrix components. It can lead to organ failure and is responsible for one-third of all deaths worldwide. However, there is no cure for this disease, and the development of minimally invasive therapies is urgently needed. We have previously reported techniques for adjusting the shape and flexibility of fibrous tissue by traction while denaturing it with heat. However, studies comparing heat and traction on fibrous tissue are limited, so this paper examined that. Applying heat and traction to bovine Achilles tendon tissue has been shown to cause the denaturation of collagen molecules to accumulate in the tissue in response to these loads. Heat-induced collagen denaturation was nondirectional and omnidirectional, whereas mechanical stress-induced collagen denaturation was concentrated in the direction of traction. When both heat and traction were applied, collagen denaturation increased more than under a single load, indicating a synergistic effect.

Denatured collagen inhibits neuroblastoma tumor-sphere migration and growth via the LOX/LOXL2 - FAK signaling pathway

A complex interplay exists within the tumor microenvironment and extracellular matrix, which could contribute to solid tumor progression. Collagen, a major component of the extracellular matrix, may correlate with cancer prognosis. While thermal ablation has shown promise as a minimally invasive treatment of solid tumors, its impact on collagen is still unknown. In this study, we demonstrate that thermal ablation, but not cryo-ablation, induces irreversible collagen denaturation in a neuroblastoma sphere model. Prolonged collagen denaturation resulted in a significant reduction in sphere stiffness, migration, and proliferation, and an increase in apoptosis. Mechanistic analysis revealed that collagen denaturation inhibited collagen cross-linking, reduced extracellular LOX/LOXL2 expression, and resulted in decreased phosphorylation of FAK. Downstream of FAK, we observed reduced epithelial to mesenchymal transition, attenuated CDC42 expression, and decreased migration. Collectively, these results suggest that denatured collagen presents a novel target for modulating the tumor microenvironment and treating solid cancers via the LOX1/LOXL2-FAK signaling pathway.

Optogenetic-induced muscle loading leads to mechanical adaptation of the Achilles tendon enthesis in mice

Skeletal shape depends on the transmission of contractile muscle forces from tendon to bone across the enthesis. Loss of muscle loading impairs enthesis development, yet little is known if and how the postnatal enthesis adapts to increased loading. Here, we studied adaptations in enthesis structure and function in response to increased loading, using optogenetically induced muscle contraction in young (i.e., growth) and adult (i.e., mature) mice. Daily bouts of unilateral optogenetic loading in young mice led to radial calcaneal expansion and warping. This also led to a weaker enthesis with increased collagen damage in young tendon and enthisis, with little change in adult mice. We then used RNA sequencing to identify the pathways associated with increased mechanical loading during growth. In tendon, we found enrichment of glycolysis, focal adhesion, and cell-matrix interactions. In bone, we found enrichment of inflammation and cell cycle. Together, we demonstrate the utility of optogenetic-induced muscle contraction to elicit in vivo adaptation of the enthesis.

Mechanobiological considerations in colorectal stapling: Implications for technology development

Technological advancements in minimally invasive surgery have led to significant improvements in patient outcomes. One such technology is surgical stapling, which has evolved into a key component of many operating rooms by facilitating ease and efficacy in resection and repair of diseased or otherwise compromised tissue. Despite such advancements, adverse post-operative outcomes such as anastomotic leak remain a persistent problem in surgical stapling and its correlates (i.e., hand-sewing), most notably in low colorectal or coloanal procedures. Many factors may drive anastomotic leaks, including tissue perfusion, microbiome composition, and patient factors such as pre-existing disease. Surgical intervention induces complex acute and chronic changes to the mechanical environment of the tissue; however, roles of mechanical forces in post-operative healing remain poorly characterized. It is well known that cells sense and respond to their local mechanical environment and that dysfunction of this "mechanosensing" phenomenon contributes to a myriad of diseases. Mechanosensing has been investigated in wound healing contexts such as dermal incisional and excisional wounds and development of pressure ulcers; however, reports investigating roles of mechanical forces in adverse post-operative gastrointestinal wound healing are lacking. To understand this relationship well, it is critical to understand: 1) the intraoperative material responses of tissue to surgical intervention, and 2) the post-operative mechanobiological response of the tissue to surgically imposed forces. In this review, we summarize the state of the field in each of these contexts while highlighting areas of opportunity for discovery and innovation which can positively impact patient outcomes in minimally invasive surgery.

Galectin-3 and RAGE differentially control advanced glycation endproduct-induced collagen damage in murine intervertebral disc organ culture

Background

Back and neck pain are leading causes of global disability that are associated with intervertebral disc (IVD) degeneration. Causes of IVD degeneration are multifactorial, and diet, age, and diabetes have all been linked to IVD degeneration. Advanced glycation endproducts (AGEs) accumulate in the IVD as a result of aging, diet, and diabetes, and AGE accumulation in the IVD has been shown to induce oxidative stress and catabolic activity that result in collagen damage. An association between AGE accumulation and IVD degeneration is emerging, yet mechanism behind this association remains unclear. The Receptor for AGEs (RAGE) is thought to induce catabolic responses in the IVD, and the AGE receptor Galectin 3 (Gal3) had a protective effect in other tissue systems but has not been evaluated in the IVD.

Methods

This study used an IVD organ culture model with genetically modified mice to analyze the roles of RAGE and Gal3 in an AGE challenge.

Results

Gal3 was protective against an AGE challenge in the murine IVD ex vivo, limiting collagen damage and biomechanical property changes. Gal3 receptor levels in the AF significantly decreased upon an AGE challenge. RAGE was necessary for AGE-induced collagen damage in the IVD, and RAGE receptor levels in the AF significantly increased upon AGE challenge.

Discussion

These findings suggest both RAGE and Gal3 are important in the IVD response to AGEs and highlight Gal3 as an important receptor with protective effects on collagen damage. This research improves understanding the mechanisms of AGE-induced IVD degeneration and suggests Gal3 receptor modulation as a potential target for preventative and therapeutic treatment for IVD degeneration.

Fatigue-driven compliance increase and collagen unravelling in mechanically tested anterior cruciate ligament

Approximately 300,000 anterior cruciate ligament (ACL) tears occur annually in the United States, half of which lead to the onset of knee osteoarthritis within 10 years of injury. Repetitive loading is known to result in fatigue damage of both ligament and tendon in the form of collagen unravelling, which can lead to structural failure. However, the relationship between tissue's structural, compositional, and mechanical changes are poorly understood. Herein we show that repetitive submaximal loading of cadaver knees causes an increase in co-localised induction of collagen unravelling and tissue compliance, especially in regions of greater mineralisation at the ACL femoral enthesis. Upon 100 cycles of 4× bodyweight knee loading, the ACL exhibited greater unravelled collagen in highly mineralized regions across varying levels of stiffness domains as compared to unloaded controls. A decrease in the total area of the most rigid domain, and an increase in the total area of the most compliant domain was also found. The results highlight fatigue-driven changes in both protein structure and mechanics in the more mineralized regions of the ACL enthesis, a known site of clinical ACL failure. The results provide a starting point for designing studies to limit ligament overuse injury.

The Chemistry and Biology of Collagen Hybridization

Collagen provides mechanical and biological support for virtually all human tissues in the extracellular matrix (ECM). Its defining molecular structure, the triple-helix, could be damaged and denatured in disease and injuries. To probe collagen damage, the concept of collagen hybridization has been proposed, revised, and validated through a series of investigations reported as early as 1973: a collagen-mimicking peptide strand may form a hybrid triple-helix with the denatured chains of natural collagen but not the intact triple-helical collagen proteins, enabling assessment of proteolytic degradation or mechanical disruption to collagen within a tissue-of-interest. Here we describe the concept and development of collagen hybridization, summarize the decades of chemical investigations on rules underlying the collagen triple-helix folding, and discuss the growing biomedical evidence on collagen denaturation as a previously overlooked ECM signature for an array of conditions involving pathological tissue remodeling and mechanical injuries. Finally, we propose a series of emerging questions regarding the chemical and biological nature of collagen denaturation and highlight the diagnostic and therapeutic opportunities from its targeting.

Optogenetic-Induced Muscle Loading Leads to Mechanical Adaptation of the Achilles Tendon Enthesis in Mice

The growth of the skeleton depends on the transmission of contractile muscle forces from tendon to bone across the extracellular matrix-rich enthesis. Loss of muscle loading leads to significant impairments in enthesis development. However, little is known about how the enthesis responds to increased loading during postnatal growth. To study the cellular and matrix adaptations of the enthesis in response to increased muscle loading, we used optogenetics to induce skeletal muscle contraction and unilaterally load the Achilles tendon and enthesis in young (i.e., during growth) and adult (i.e., mature) mice. In young mice, daily bouts of unilateral optogenetic loading led to expansion of the calcaneal apophysis and growth plate, as well as increased vascularization of the normally avascular enthesis. Daily loading bouts, delivered for 3 weeks, also led to a mechanically weaker enthesis with increased molecular-level accumulation of collagen damage in young mice. However, adult mice did not exhibit impaired mechanical properties or noticeable structural adaptations to the enthesis. We then focused on the transcriptional response of the young tendon and bone following optogenetic-induced loading. After 1 or 2 weeks of loading, we identified, in tendon, transcriptional activation of canonical pathways related to glucose metabolism (glycolysis) and inhibited pathways associated with cytoskeletal remodeling (e.g., RHOA and CREB signaling). In bone, we identified activation of inflammatory signaling (e.g., NFkB and STAT3 signaling) and inhibition of ERK/MAPK and PTEN signaling. Thus, we have demonstrated the utility of optogenetic-induced skeletal muscle contraction to elicit structural, functional, and molecular adaptation of the enthesis in vivo especially during growth.

Strain-induced collagen denaturation is rate dependent in failure of cerebral arteries

While soft tissues are commonly damaged by mechanical loading, the manifestation of this damage at the microstructural level is not fully understood. Specifically, while rate-induced stiffening has been previously observed in cerebral arteries, associated changes in microstructural damage patterns following high-rate loading are largely undefined. In this study, we stretched porcine middle cerebral arteries to failure at 0.01 and >150 s^-1, both axially and circumferentially, followed by probing for denatured tropocollagen using collagen hybridizing peptide (CHP). We found that collagen fibrils aligned with the loading direction experienced less denaturation following failure tests at high than low rates. Others have demonstrated similar rate dependence in tropocollagen denaturation during soft tissue failure, but this is the first study to quantify this behavior using CHP and to report it for cerebral arteries. These findings may have significant implications for traumatic brain injury and intracranial balloon angioplasty. We additionally observed possible tropocollagen denaturation in vessel layers primarily composed of fibrils transversely aligned to the loading axis. To our knowledge, this is the first observation of collagen denaturation due to transverse loading, but further research is needed to confirm this finding. STATEMENT OF SIGNIFICANCE: Previous work shows that collagen hybridizing peptide (CHP) can be used to identify collagen molecule unfolding and denaturation in mechanically overloaded soft tissues, including the cerebral arteries. But experiments have not explored collagen damage at rates relevant to traumatic brain injury. In this work, we quantified collagen damage in cerebral arteries stretched to failure at both high and low rates. We found that the collagen molecule is less damaged at high than at low rates, suggesting that damage mechanisms of either the collagen molecule or other elements of the collagen superstructure are rate dependent. This work implies that arteries failed at high rates, such as in traumatic brain injury, will have different molecular-level damage patterns than arteries failed at low rates. Consequently, improved understanding of damage characteristics may be expanded in the future to better inform clinically relevant cases of collagen damage such as angioplasty and injury healing.

Mouse Achilles tendons exhibit collagen disorganization but minimal collagen denaturation during cyclic loading to failure

While overuse is a prominent risk factor for tendinopathy, the fatigue-induced structural damage responsible for initiating tendon degeneration remains unclear. Denaturation of collagen molecules and collagen fiber disorganization have been observed within certain tendons in response to fatigue loading. However, no studies have investigated whether these forms of tissue damage occur in Achilles tendons, which commonly exhibit tendinopathy. Therefore, the objective of this study was to determine whether mouse Achilles tendons undergo collagen denaturation and collagen fiber disorganization when cyclically loaded to failure. Consistent with previous testing of other energy-storing tendons, we found that cyclic loading of mouse Achilles tendons produced collagen disorganization but minimal collagen denaturation. To determine whether the lack of collagen denaturation is unique to mouse Achilles tendons, we monotonically loaded the Achilles and other mouse tendons to failure. We found that the patellar tendon was also resistant to collagen denaturation, but the flexor digitorum longus (FDL) tendon and tail tendon fascicles were not. Furthermore, the Achilles and patellar tendons had a lower tensile strength and modulus. While this may be due to differences in tissue structure, it is likely that the lack of collagen denaturation during monotonic loading in both the Achilles and patellar tendons was due to failure near their bony insertions, which were absent in the FDL and tail tendons. These findings suggest that mouse Achilles tendons are resistant to collagen denaturation in situ and that Achilles tendon degeneration may not be initiated by mechanically-induced damage to collagen molecules.

Collagen Molecular Damage is a Hallmark of Early Atherosclerosis Development

Remodeling of extracellular matrix proteins underlies the development of cardiovascular disease. Herein, we utilized a novel molecular probe, collagen hybridizing peptide (CHP), to target collagen molecular damage during atherogenesis. The thoracic aorta was dissected from ApoE-/- mice that had been on a high-fat diet for 0-18 weeks. Using an optimized protocol, tissues were stained with Cy3-CHP and digested to quantify CHP with a microplate assay. Results demonstrated collagen molecular damage, inferred from Cy3-CHP fluorescence, was a function of location and time on the high-fat diet. Tissue from the aortic arch showed a significant increase in collagen molecular damage after 18 weeks, while no change was observed in tissue from the descending aorta. No spatial differences in fluorescence were observed between the superior and inferior arch tissue. Our results provide insight into the early changes in collagen during atherogenesis and present a new opportunity in the subclinical diagnosis of atherosclerosis.

Controlling the Trimerization of the Collagen Triple-Helix by Solvent Switching

Collagen hybridizing peptides (CHPs) are a powerful tool for targeting collagen damage in pathological tissues due to their ability to specifically form a hybrid collagen triple-helix with the denatured collagen chains. However, CHPs have a strong tendency to self-trimerize, requiring preheating or complicated chemical modifications to dissociate their homotrimers into monomers, which hinders their applications. To control the self-assembly of CHP monomers, we evaluated the effects of 22 cosolvents on the triple-helix structure: unlike typical globular proteins, the CHP homotrimers (as well as the hybrid CHP-collagen triple helix) cannot be destabilized by the hydrophobic alcohols and detergents (e.g., SDS) but can be effectively dissociated by the cosolvents that dominate hydrogen bonds (e.g., urea, guanidinium salts, and hexafluoroisopropanol). Our study provided a reference for the solvent effects on natural collagen and a simple effective solvent-switch method, enabling CHP utilization in automated histopathology staining and in vivo imaging and targeting of collagen damage.

Finite element modeling of meniscal tears using continuum damage mechanics and digital image correlation

Meniscal tears are a common, painful, and debilitating knee injury with limited treatment options. Computational models that predict meniscal tears may help advance injury prevention and repair, but first these models must be validated using experimental data. Here we simulated meniscal tears with finite element analysis using continuum damage mechanics (CDM) in a transversely isotropic hyperelastic material. Finite element models were built to recreate the coupon geometry and loading conditions of forty uniaxial tensile experiments of human meniscus that were pulled to failure either parallel or perpendicular to the preferred fiber orientation. Two damage criteria were evaluated for all experiments: von Mises stress and maximum normal Lagrange strain. After we successfully fit all models to experimental force-displacement curves (grip-to-grip), we compared model predicted strains in the tear region at ultimate tensile strength to the strains measured experimentally with digital image correlation (DIC). In general, the damage models underpredicted the strains measured in the tear region, but models using von Mises stress damage criterion had better overall predictions and more accurately simulated experimental tear patterns. For the first time, this study has used DIC to expose strengths and weaknesses of using CDM to model failure behavior in soft fibrous tissue.

Designing Collagen-Binding Peptide with Enhanced Properties Using Hydropathic Free Energy Predictions

Collagen is fundamental to a vast diversity of health functions and potential therapeutics. Short peptides targeting collagen are attractive for designing modular systems for site-specific delivery of bioactive agents. Characterization of peptide-protein binding involves a larger number of potential interactions that require screening methods to target physiological conditions. We build a hydropathy-based free energy estimation tool which allows quick evaluation of peptides binding to collagen. Previous studies showed that pH plays a significant role in collagen structure and stability. Our design tool enables probing peptides for their collagen-binding property across multiple pH conditions. We explored binding features of currently known collagen-binding peptides, collagen type I alpha chain 2 sense peptide (TKKTLRT) and decorin LRR-10 (LRELHLNNN). Based on these analyzes, we engineered a collagen-binding peptide with enhanced properties across a large pH range in contrast to LRR-10 pH dependence. To validate our predictions, we used a quantum-dots-based binding assay to compare the coverage of the peptides on type I collagen. The predicted peptide resulted in improved collagen binding. Hydropathy of the peptide-protein pair is a promising approach to finding compatible pairings with minimal use of computational resources, and our method allows for quick evaluation of peptides for binding to other proteins. Overall, the free-energy-based tool provides an alternative computational screening approach that impacts protein interaction search methods.

Global and local stiffening of ex vivo-perfused stented human thoracic aortas: A mock circulation study

The effects of thoracic endovascular repair (TEVAR) on the biomechanical properties of aortic tissue have not been adequately studied. Understanding these features is important for the management of endograft-triggered complications of a biomechanical nature. This study aims to examine how stent-graft implantation affects the elastomechanical behavior of the aorta. Non-pathological human thoracic aortas (n=10) were subjected to long-standing perfusion (8h) within a mock circulation loop under physiological conditions. To quantify compliance and its mismatch in the test periods without and with a stent, the aortic pressure and the proximal cyclic circumferential displacement were measured. After perfusion, biaxial tension tests (stress-stretch) were carried out to examine the stiffness profiles between non-stented and stented tissue, followed by a histological assessment. Experimental evidence shows: (i) a significant reduction in aortic distensibility after TEVAR, indicating aortic stiffening and compliance mismatch, (ii) a stiffer behavior of the stented samples compared to the non-stented samples with an earlier entry into the nonlinear part of the stress-stretch curve and (iii) strut-induced histological remodeling of the aortic wall. The biomechanical and histological comparison of the non-stented and stented aortas provides new insights into the interaction between the stent-graft and the aortic wall. The knowledge gained could refine the stent-graft design to minimize the stent-induced impacts on the aortic wall and the resulting complications. STATEMENT OF SIGNIFICANCE: Stent-related cardiovascular complications occur the moment the stent-graft expands on the human aortic wall. Clinicians base their diagnosis on the anatomical morphology of CT scans while neglecting the endograft-triggered biomechanical events that compromise aortic compliance and wall mechanotransduction. Experimental replication of endovascular repair in cadaver aortas within a mock circulation loop may have a catalytic effect on biomechanical and histological findings without an ethical barrier. Demonstrating interactions between the stent and the wall can help clinicians make a broader diagnosis such as ECG-triggered oversizing and stent-graft characteristics based on patient-specific anatomical location and age. In addition, the results can be used to optimize towards more aortophilic stent grafts.