Sugars and polyols inhibit fibrillogenesis of type I collagen by disrupting hydrogen-bonded water bridges between the helices

Interfacial water on collagen nanoribbons by 3D AFM

Collagen is the most abundant structural protein in mammals. Type I collagen in its fibril form has a characteristic pattern structure that alternates two regions called gap and overlap. The structure and properties of collagens are highly dependent on the water and mineral content of the environment. Here, we apply 3D AFM to characterize at angstrom-scale resolution the interfacial water structure of collagen nanoribbons. For a neutral tip, the interfacial water structure is characterized by the oscillation of the water particle density distribution with a value of 0.3 nm (hydration layers). The interfacial structure does not depend on the collagen region. For a negatively charged tip, the interfacial structure might depend on the collagen region. Hydration layers are observed in overlap regions, while in gap regions, the interfacial solvent structure is dominated by electrostatic interactions. These interactions generate interlayer distances of 0.2 nm.

Structural and mechanical behavior of type-I collagen fibrils in presence of induced electrostatic interactions through ionic liquids

Tuning the self-assembly of collagen has broad applications in the biomedical field owing to their desired biological performance as collagenous materials with tunable functionalities can further determine cellular responses. In this work, an attempt has been made to tune the self-assembly of collagen using ionic liquids, viz., imidazolium chloride (IC) and choline dihydrogen phosphate (CDHP) at its physiological pH, followed by probing assembled systems using various characterization methods. Turbidity measurements of fibrillar networks were performed to ascertain the rate of fibril formation in addition of imidazolium chloride and choline dihydrogen phosphate to collagen at physiological pH. Morphological changes were examined using Scanning Electron Microscope (SEM), binding affinities were measured by Microscale Thermophoresis (MST), in addition to, changes in the shear viscosity, mechanical strength of collagen fibrils when interacted with imidazolium and choline based ILs were carried out using rotational rheometer and Quartz Crystal Microbalance (QCM) measurements. Experimental result depicts that CDHP imparts better crosslinking as well as mechanical strength compare to IC, which is already known for destabilizing the triple helix structure is inhibiting the fibril formation. This self-assembled, ionic-liquid treated collagen-fibrillar system would accelerate various force modulated fibrillar network study, for mimicking the ECM and tissue engineering application.

DMSO synergized with sugars: optical clearing agent modulation of optical coherence tomography of skin tissues

The stratum corneum of the outermost skin is an important barrier impeding transdermal permeation, and permeation enhancers can reduce the barrier resistance of the stratum corneum and enhance the permeation of drugs in tissues. The optical imaging depth, signal intensity, and scattering coefficient variation rules of skin tissues in time dimension are obtained by using optical coherence tomography (OCT). The effect of optical clearing agents (OCAs) on OCT imaging is obtained by quantitatively analyzing the changes in the optical properties of tissues. D-fructose, one of the monosaccharides, and sucrose, one of the disaccharides, were selected for the ex vivo optical clearing experiments on pig skin tissues utilizing the dimethyl sulfoxide (DMSO) carrier effect. We find that DMSO synergized with sugars applied to skin tissue has a more significant increase in the optical imaging depth and signal intensity, and a reduction in the scattering coefficient with an increasing concentration of DMSO. DMSO with a high concentration and D-fructose with saturated concentration (10:1; v/v) effectively reduce light attenuation in OCT imaging and improve the image quality. This operation will also shorten the application time to minimize skin damage from hyperosmotic agents.

Fluence-dependent degradation of fibrillar type I collagen by 222 nm far-UVC radiation

For more than 100 years, germicidal lamps emitting 254 nm ultraviolet (UV) radiation have been used for drinking-water disinfection and surface sterilization. However, due to the carcinogenic nature of 254 nm UV, these lamps have been unable to be used for clinical procedures such as wound or surgical site sterilization. Recently, technical advances have facilitated a new generation of germicidal lamp whose emissions centre at 222 nm. These novel 222 nm lamps have commensurate antimicrobial properties to 254 nm lamps while producing few short- or long-term health effects in humans upon external skin exposure. However, to realize the full clinical potential of 222 nm UV, its safety upon internal tissue exposure must also be considered. Type I collagen is the most abundant structural protein in the body, where it self-assembles into fibrils which play a crucial role in connective tissue structure and function. In this work, we investigate the effect of 222 nm UV radiation on type I collagen fibrils in vitro. We show that collagen's response to irradiation with 222 nm UV is fluence-dependent, ranging from no detectable fibril damage at low fluences to complete fibril degradation and polypeptide chain scission at high fluences. However, we also show that fibril degradation is significantly attenuated by increasing collagen sample thickness. Given the low fluence threshold for bacterial inactivation and the macroscopic thickness of collagenous tissues in vivo, our results suggest a range of 222 nm UV fluences which may inactivate pathogenic bacteria without causing significant damage to fibrillar collagen. This presents an initial step toward the validation of 222 nm UV radiation for internal tissue disinfection.

Pre-glycation impairs gelation of high concentration collagen solutions

There remains a need for stiffer collagen hydrogels for tissue engineering and disease modeling applications. Pre-glycation, or glycation of collagen in solution prior to gelation, has been shown to increase the mechanics of collagen hydrogels while maintaining high viability of encapsulated cells. The stiffness of glycated collagen gels can be increased by increasing the collagen concentration, sugar concentration, and glycation time. However, previous studies on pre-glycation of collagen have used low collagen concentrations and/or low sugar concentrations and have not investigated the effect of glycation time. Therefore, the objective of this study was to determine the effects of pre-glycation with high sugar concentrations (up to 500 mM) and extended glycation times (up to 21 days) on high concentration collagen (8 mg/ml). The addition of sugar to the collagen and the formation of advanced glycation end products (AGEs) were quantified. The ability to gel successfully and rheological properties were determined and correlated with biochemical characterizations. Successful collagen gelation and rheological properties of pre-glycated collagen were found to be strongly dependent on the ratio of added sugars to added AGEs with high ratios impairing gelation and low ratios resulting in optimal storage moduli. There is likely a competing effect during pre-glycation of the formation of AGEs resulting in crosslinking of collagen and the formation of Amadori intermediates acting to increase collagen solubility. Overall, this study shows that collagen glycation can be optimized by increasing the formation of AGEs while maintaining a low ratio of added sugar to added AGEs.

Incorporation of a Collagen-Binding Chondroitin Sulfate Molecule to a Collagen Type I and II Blend Hydrogel for Cartilage Tissue Engineering

Adding chondroitin sulfate (CS) to collagen scaffolds has been shown to improve the outcomes for articular cartilage tissue engineering. Instead of physical entrapment or chemical crosslinking of CS within a scaffold, this study investigated the use of CS with attached collagen-binding peptides (termed CS-SILY). This method better recapitulates the aspects of native cartilage while retaining CS within a collagen type I and II blend (Col I/II) hydrogel. CS retention, average fibril diameter, and mechanical properties were altered by varying the number of SILY peptides attached to the CS backbone. When mesenchymal stromal cells (MSCs) were encapsulated within the scaffolds, the addition of CS-SILY molecules resulted in higher sulfated glycosaminoglycan production, and these results suggest that CS-SILY promotes MSC differentiation into chondrocytes. Taken together, our study shows the promise of adding a CS-SILY molecule to a Col I/II hydrogel with encapsulated MSCs to promote cartilage repair.

Polyanion-induced, microfluidic engineering of fragmented collagen microfibers for reconstituting extracellular environments of 3D hepatocyte culture

Artificial biological scaffolds made of extracellular matrix (ECM) components, such as type I collagen, provide ideal physicochemical cues to various cell culture platforms. However, it remains a challenge to fabricate micrometer-sized ECM materials with precisely controlled morphologies that could reconstitute the 3-dimensional (3D) microenvironments surrounding cells. In the present study, we proposed a unique process to fabricate fragmented collagen microfibers using a microfluidic laminar-flow system. The continuous flow of an acidic collagen solution was neutralized to generate solid fibers, which were subsequently fragmented by applying a gentle shear stress in a polyanion-containing phosphate buffer. The morphology of the fiber fragment was controllable in a wide range by changing the type and/or concentration of the polyanion and by tuning the applied shear stress. The biological benefits of the fragmented fibers were investigated through the formation of multicellular spheroids composed of primary rat hepatocytes and microfibers on non-cell-adhesive micro-vessels. The microfibers enhanced the survival and functions of the hepatocytes and reproduced proper cell polarity, because the fibers facilitated the formation of cell-cell and cell-matrix interactions while modulating the close packing of cells. These results clearly indicated that the microengineered fragmented collagen fibers have great potential to reconstitute extracellular microenvironments for hepatocytes in 3D culture, which will be of significant benefit for cell-based drug testing and bottom-up tissue engineering.

Physical and chemical mechanisms of tissue optical clearing

Advanced optical methods combined with various probes pave the way toward molecular imaging within living cells. However, major challenges are associated with the need to enhance the imaging resolution even further to the subcellular level for the imaging of larger tissues, as well as for in vivo studies. High scattering and absorption of opaque tissues limit the penetration of light into deep tissues and thus the optical imaging depth. Tissue optical clearing technique provides an innovative way to perform deep-tissue imaging. Recently, various optical clearing methods have been developed, which provide tissue clearing based on similar physical principles via different chemical approaches. Here, we introduce the mechanisms of the current clearing methods from fundamental physical and chemical perspectives, including the main physical principle, refractive index matching via various chemical approaches, such as dissociation of collagen, delipidation, decalcification, dehydration, and hyperhydration, to reduce scattering, as well as decolorization to reduce absorption.

Tendon Extracellular Matrix Assembly, Maintenance and Dysregulation Throughout Life

In his Lissner Award medal lecture in 2000, Stephen Cowin asked the question: "How is a tissue built?" It is not a new question, but it remains as relevant today as it did when it was asked 20 years ago. In fact, research on the organization and development of tissue structure has been a primary focus of tendon and ligament research for over two centuries. The tendon extracellular matrix (ECM) is critical to overall tissue function; it gives the tissue its unique mechanical properties, exhibiting complex non-linear responses, viscoelasticity and flow mechanisms, excellent energy storage and fatigue resistance. This matrix also creates a unique microenvironment for resident cells, allowing cells to maintain their phenotype and translate mechanical and chemical signals into biological responses. Importantly, this architecture is constantly remodeled by local cell populations in response to changing biochemical (systemic and local disease or injury) and mechanical (exercise, disuse, and overuse) stimuli. Here, we review the current understanding of matrix remodeling throughout life, focusing on formation and assembly during the postnatal period, maintenance and homeostasis during adulthood, and changes to homeostasis in natural aging. We also discuss advances in model systems and novel tools for studying collagen and non-collagenous matrix remodeling throughout life, and finally conclude by identifying key questions that have yet to be answered.

Insights into the effect of artificial sweeteners on the structure, stability, and fibrillation of type I collagen

Artificial sweeteners (AS) are widely used as sugar substitutes because natural sweetener (sugar) leads to a number of health issues, including diabetes, obesity, and tooth decay. Since natural sugar (sucrose), diabetes and skin are highly interlinked, and also sucrose is known to inhibit the fibrillation of collagen, the major protein of the skin, a study on the impact of AS on collagen is important and essential. Herein, we have studied the influence of commonly used AS such as Sucralose (SUC), Aspartame (APM), and Saccharin (SAC) on the structure, stability, and fibrillation of collagen using various spectroscopic methods. The circular dichroism and turbidity results suggest that the AS does not disrupt the triple helix structure and also the fibrillar property of collagen, respectively. The fibrillar morphology was sustained, although there was a trivial difference in the entanglement of fibrils in the presence of SAC, compared to native collagen fibrils. The thermal stability of collagen is maintained in the presence of AS. Fluorescence and STD-NMR results indicate that the interaction between AS and collagen was weak, which supports the intact structure, stability, and fibrillation property of collagen. The current study thus suggests that the chosen AS does not influence collagen properties.

Defective chicken skin collagen molecules, hydrolyzed by actinidain protease, assemble to form loosely packed fibrils that promote cell spheroid formation

Cells recognize collagen fibrils as the first step in the process of adherence. Fibrils of chicken skin actinidain-hydrolyzed collagen (low adhesive scaffold collagen, LASCol), in which the telopeptide domains are almost completely removed, cause adhering cells to form spheroids instead of adopting a monolayer morphology. Our goal was to elucidate the ultrastructure of the LASCol fibrils compared with pepsin-hydrolyzed collagen (PepCol) fibrils. At low concentration of 0.2 mg/mL, the time to reach the maximum increasing rate of turbidity for LASCol was all slower than that for PepCol. Differential scanning calorimetry showed that the thermal stability of collagen self-assembly changed significantly between pH 5.5 and pH 6.6 with and without a small number of telopeptides. However, the calorimetric enthalpy change did not vary much in that pH range. The melting temperature of LASCol fibrils at pH 7.3 was 55.1 °C, whereas PepCol fibrils exhibited a peak around 56.9 °C. The D-periodicity of each fibril was the same at 67 nm. Nevertheless, the looseness of molecular packing in LASCol fibrils was demonstrated by circular dichroism measurements and immuno-scanning electron microscopy with a polyclonal antibody against type I collagen. As there is a close relationship between function and structure, loosely packed collagen fibrils would be one factor that promotes cell spheroid formation.

Biophysical Elucidation of Fibrillation Inhibition by Sugar Osmolytes in α-Lactalbumin: Multispectroscopic and Molecular Docking Approaches

Protein aggregation is among the most challenging new frontiers in protein chemistry as well as in molecular medicine and has direct implications in protein misfolding. This study investigated the role of sugar molecules (glucose, fructose, sucrose, and the mixture of glucose and fructose) in protecting the structural integrity of α-lactalbumin (α-LA) against aggregation. The research focused here is the inhibitory capabilities of sugars against α-LA fibril formation investigated employing diverse multispectroscopic and microscopic techniques. The aggregation was induced in α-LA thermally with a change in concentration. UV-vis spectroscopy, ThT binding assay, Trp fluorescence, Rayleigh scattering, and turbidity assay depicted synchronized results. Further, transmission electron microscopy (TEM) complemented that a mixture of glucose and fructose was the best inhibitor of α-LA fibril formation. Inhibition of α-LA aggregation by sugar osmolytes is attributed to the formation of hydrogen bonds between these osmolytes, as evidenced by the molecular docking results. This hydrogen bonding is a key player that prevents aggregation in α-LA in the presence of sugar osmolytes. This study provides an insight into the ability of naturally occurring sugar osmolytes to inhibit fibril formation and can serve as a platform to treat protein misfolding and aggregation-oriented disorders.

Profiling of drug crystallization in the skin

BACKGROUND

Drug crystallization following application of transdermal and topical formulations may potentially compromise the delivery of drugs to the skin. This phenomenon was found to be limited to the superficial layers of the stratum corneum (~7 µm) in our recent reports and tape stripping of the skin samples was necessary. It remains a significant challenge to profile drug crystallization in situ without damaging the skin samples.

METHODS

This work reports the application of an X-ray microbeam via synchrotron SAXS/WAXS analysis to monitor drug crystallization in the skin, especially in the deeper skin layers. Confocal Raman spectroscopy (CRS) was employed to examine drug distribution in the skin to complement the detection of drug crystallization using SAXS/WAXS analysis.

RESULTS

Following application of saturated drug solutions (ibuprofen, diclofenac acid, and salts), CRS depth profiles confirmed that the drugs generally were delivered to a depth of ~15 - 20 µm in the skin. This was compared with the WAXS profiles that measured drug crystal diffraction at a depth of up to ~25 µm of the skin.

CONCLUSION

This study demonstrates the potential of synchrotron SAXS/WAXS analysis for profiling of drug crystallization in situ in the deeper skin layers without pre-treatment for the skin samples. [Figure: see text].

Mast cells and collagen fibrillogenesis

This article presents 20 combinations of histochemical stainings for the determination of mast cell co-localization with the fibrous component of the connective tissue in the fibrillogenesis course. Best results were obtained using metachromatic detection of mast cells in combination with silver or picro-fuchsin impregnation, staining with brilliant green using van Gieson staining, and a combination of aniline blue staining with neutral red. Proposed variants of histochemical protocols open up new opportunities to analyze the participation of mast cells in extracellular matrix remodeling of the tissue microenvironment in the course of adaptive and pathological processes. Results obtained expand the current theoretical views of the process of fibrillogenesis in the extracellular matrix. They also shed new light on the participation of mast cell secretion components in the molecular mechanisms of fiber formation.

Extracellular matrix-mimetic composite hydrogels of cross-linked hyaluronan and fibrillar collagen with tunable properties and ultrastructure

A platform of enzymatically-crosslinked Collagen/Tyramine hyaluronan derivative (Col/HA-Tyr) hydrogels with tunable compositions and gelation conditions was developed to evaluate the impact of the preparation conditions on their physical, chemical and biological properties. At low HA-Tyr content, hydrogels exhibited a fibrillar structure, with lower mechanical properties compared to pure Col hydrogels. At high HA-Tyr and Horse Radish Peroxydase (HRP) content, a microfibrillar network was formed beside the banded Col fibrils and a synergistic effect of the hybrid structure on mechanical properties was observed. These hydrogels were highly resistant against enzymatic degradation while keeping a high degree of hydration. Unlike HA-Tyr hydrogels, encapsulation of human dermal fibroblasts within Col/HA-Tyr hydrogels allowed for high cell viability. These results showed that high HA-Tyr and HRP concentrations are required to positively impact the physical properties of hydrogels while preserving collagen fibrils. Those Col/HA-Tyr hydrogels appear promising for novel tissue engineering applications following a biomimetic approach.

Factors associated with obesity alter matrix remodeling in breast cancer tissues

Obesity is associated with a higher risk of developing breast cancer and with worse disease outcomes for women of all ages. The composition, density, and organization of the breast tissue stroma are also known to play an important role in the development and progression of the disease. However, the connections between obesity and stromal remodeling are not well understood. We sought to characterize detailed organization features of the collagen matrix within healthy and cancerous breast tissues acquired from mice exposed to either a normal or high fat (obesity inducing) diet. We performed second-harmonic generation and spectral two-photon excited fluorescence imaging, and we extracted the level of collagen-associated fluorescence (CAF) along with metrics of collagen content, three-dimensional, and two-dimensional organization. There were significant differences in the CAF intensity and overall collagen organization between normal and tumor tissues; however, obesity-enhanced changes in these metrics, especially when three-dimensional organization metrics were considered. Thus, our studies indicate that obesity impacts significantly collagen organization and structure and the related pathways of communication may be important future therapeutic targets.

Self-association of type I collagen directed by thymoquinone through alteration of molecular forces

Type I collagen is a vital structural component of the extracellular matrix providing the connective tissues with biomechanical support. One of the interesting properties of collagen is to self-associate into fibrils. The present work aims to direct the self-assembly of collagen through different molecular forces, which are tuned on the addition of thymoquinone a well-known phytochemical. A change in relative viscosity and stress of collagen-thymoquinone blends influenced the interfibrillar aggregates around its hydration shell. Further, secondary structural integrity was studied via cotton curve effect, and vibrational frequency shifts showed a characteristic interaction of thymoquinone at the N-terminal residues of the triple helix. Finally, the spontaneous self-association of fibrils was tracked by calculating the rate of fibril growth kinetics, which potentially decreased with increase in thymoquinone concentration. The fibrils were eventually visualized under the high resolution-scanning microscope showing morphological variations. Therefore, such a protein-phytochemical interaction may tend to play with the hydration network of collagen and covalently interact with its imino acid residues. It may be speculated that such an inhibitory process portrayed by thymoquinone may have a fortune in the targeted and sustainable delivery to the site of action for certain diseases, which includes collagen accumulation. Moreover, its directed assembly could be utilized for designing templates as in manipulating the collagen as a nanoporous membrane to make nanofibers and further tuned by small molecules for nanoparticle synthesis application.

Strategically Designed Antifibrotic Gold Nanoparticles to Prevent Collagen Fibril Formation

Because uncontrolled accumulation of collagen fibrils has been implicated in a series of pathologies, inhibition of collagen fibril formation has become one of the necessary strategies to target such collagen-linked complications. The presence of hydroxyproline (Hyp) at the Y position in (Gly-X-Y)_n sequence pattern of collagen is known to facilitate crucial hydrophobic and hydration-linked interactions that promote collagen fibril formation. Here, to target such Hyp-mediated interactions, we have synthesized uniform, thermostable, and hemocompatible Hyp coated gold nanoparticles (AuNPs^HYP) and have examined their inhibition effect on the fibril formation of type I collagen. We found that collagen fibril formation is strongly suppressed in the presence of AuNPs^HYP and no such suppression effect was observed in the presence of free Hyp and control Gly-coated nanoparticles at similar concentrations. Both isothermal titration calorimetric studies and bioinformatics analysis reveal possible interaction between Hyp and (Gly-Pro-Hyp) stretches of collagen triple-helical model peptides. Further, gold nanoparticles coated with proline (AuNPs^PRO) and tryptophan (AuNPs^TRP) also suppressed collagen fibril formation, suggesting their ability to interfere with aromatic-proline as well as hydrophobic interactions between collagen molecules. The Hyp molecules, when surface functionalized, are predicted to interfere with the Hyp-mediated forces that drive collagen self-assembly, and such inhibition effect may help in targeting collagen linked pathologies.

Modulation of Matrix Softness and Interstitial Flow for 3D Cell Culture Using a Cell-Microenvironment-on-a-Chip System

In the past several decades, significant efforts have been devoted to recapitulating the in vivo tissue microenvironment within an in vitro platform. However, it is still challenging to recreate de novo tissue with physiologically relevant matrix properties and fluid flow. To this end, this study demonstrates a method to independently tailor matrix stiffness and interstitial fluid flow using a cell-microenvironment-on-a-chip (C-MOC) platform. Collagen-polyethylene glycol gels tailored to present controlled stiffness and hydraulic conductivity were fabricated in a microfluidic chip. The chip was assembled to continuously create a steady flow of media through the gel. In the C-MOC platform, interstitial flow mitigated the effects of matrix softness on breast cancer cell behavior, according to an immunostaining-based analysis of estrogen receptor-α (ER-α), integrin β1, and E-cadherin. This advanced cell culture platform serves to engineer tissue similar to in vitro tissue and contribute to better understanding and regulating of the biological roles of extracellular microenvironments.

Combined effects of solvation and aggregation propensity on the final supramolecular structures adopted by hydrophobic, glycine-rich, elastin-like polypeptides

Previous work on elastin-like polypeptides (ELPs) made of hydrophobic amino acids of the type XxxGlyGlyZzzGly (Xxx, Zzz = Val, Leu) has consistently shown that differing dominant supramolecular structures were formed when the suspending media were varied: helical, amyloid-like fibers when suspended in water and globules evolving into "string of bead" structures, poly(ValGlyGlyValGly), or cigar-like bundles, poly(ValGlyGlyLeuGly), when suspended in methyl alcohol. Comparative experiments with poly(LeuGlyGlyValGly) have further indicated that the interface energy plays a significant role and that solvation effects act in concomitance with the intrinsic aggregation propensity of the repeat sequence. Continuing our investigation on ELPs using surface (X-ray photoelectron spectroscopy, atomic force microscopy) and bulk (circular dichroism, Fourier transform infrared spectroscopy) techniques for their characterization, here we have compared the effect of suspending solvents (H(2)O, dimethylsulfoxide, ethylene glycol, and MeOH) on poly(ValGlyGlyValGly), the polypeptide most inclined to form long and well-refined helical fibers in water, searching for the signature of intermolecular interactions occurring between the polypeptide chains in the given suspension. The influence of sequence specificities has been studied by comparing poly(ValGlyGlyValGly) and poly(LeuGlyGlyValGly) with a similar degree of polymerization. Deposits on substrates of the polypeptides were characterized taking into account the differing evaporation rate of solvents, and tests on their stability in ultra high vacuum were performed. Finally, combining experimental and computational studies, we have revaluated the three-dimensional modeling previously proposed for the supramolecular assembly in water of poly(ValGlyGlyValGly). The results were discussed and rationalized also in the light of published data.

Fluorescent nanonetworks: a novel bioalley for collagen scaffolds and tissue engineering

Native collagen is arranged in bundles of aligned fibrils to withstand in vivo mechanical loads. Reproducing such a process under in vitro conditions has not met with major success. Our approach has been to induce nanolinks, during the self-assembly process, leading to delayed rather than inhibited fibrillogenesis. For this, a designed synthesis of nanoparticles - using starch as a template and a reflux process, which would provide a highly anisotropic (star shaped) nanoparticle, with large surface area was adopted. Anisotropy associated decrease in Morin temperature and superparamagnetic behavior was observed. Polysaccharide on the nanoparticle surface provided aqueous stability and low cytotoxicity. Starch coated nanoparticles was utilized to build polysaccharide - collagen crosslinks, which supplemented natural crosslinks in collagen, without disturbing the conformation of collagen. The resulting fibrillar lamellae showed a striking resemblance to native lamellae, but had a melting and denaturation temperature higher than native collagen. The biocompatibility and superparamagnetism of the nanoparticles also come handy in the development of stable collagen constructs for various biomedical applications, including that of MRI contrast agents.

Strain-induced optical changes in demineralized bone

Bone "stress-whitens," becoming visibly white during mechanical loading, immediately prior to failure. Stress-whitening is known to make materials tougher by dissipating mechanical energy. A greater understanding of stress-whitening, both an optical and mechanical phenomenon, may help explain age-related increases in fracture risk that occur without changes in bone mineralization. In this work, we directly measure the optical properties of demineralized bone as a function of deformation and immersing fluid (with different hydrogen-bonding potentials, water, and ethanol). The change in refractive index of demineralized bone was linear: with deformation and not applied force. Changes in refractive index were likely due to pushing low-refractive-index fluid out of specimens and secondarily due to changes in the refractive index of the collagenous phase. Results were consistent with stress-whitening of demineralized bone previously observed. In ethanol, the refractive index values were lower and less sensitive to deformation compared with deionized water, corroborating the sensitivity to fluid hydration. Differences in refractive index were consistent with structural changes in the collagenous phase such as densification that may also occur under mechanical loading. Understanding bone quality, particularly stress-whitening investigated here, may lead to new therapeutic targets and noninvasive methods to assess bone quality.

Thermal memory in self-assembled collagen fibril networks

Collagen fibrils form extracellular networks that regulate cell functions and provide mechanical strength to tissues. Collagen fibrillogenesis is an entropy-driven process promoted by warming and reversed by cooling. Here, we investigate the influence of noncovalent interactions mediated by the collagen triple helix on fibril stability. We measure the kinetics of cold-induced disassembly of fibrils formed from purified collagen I using turbimetry, probe the fibril morphology by atomic force microscopy, and measure the network connectivity by confocal microscopy and rheometry. We demonstrate that collagen fibrils disassemble by subunit release from their sides as well as their ends, with complex kinetics involving an initial fast release followed by a slow release. Surprisingly, the fibrils are gradually stabilized over time, leading to thermal memory. This dynamic stabilization may reflect structural plasticity of the collagen fibrils arising from their complex structure. In addition, we propose that the polymeric nature of collagen monomers may lead to slow kinetics of subunit desorption from the fibril surface. Dynamic stabilization of fibrils may be relevant in the initial stages of collagen assembly during embryogenesis, fibrosis, and wound healing. Moreover, our results are relevant for tissue repair and drug delivery applications, where it is crucial to control fibril stability.

Polarizable empirical force field for acyclic polyalcohols based on the classical Drude oscillator

A polarizable empirical force field for acyclic polyalcohols based on the classical Drude oscillator is presented. The model is optimized with an emphasis on the transferability of the developed parameters among molecules of different sizes in this series and on the condensed-phase properties validated against experimental data. The importance of the explicit treatment of electronic polarizability in empirical force fields is demonstrated in the cases of this series of molecules with vicinal hydroxyl groups that can form cooperative intra- and intermolecular hydrogen bonds. Compared to the CHARMM additive force field, improved treatment of the electrostatic interactions avoids overestimation of the gas-phase dipole moments resulting in significant improvement in the treatment of the conformational energies and leads to the correct balance of intra- and intermolecular hydrogen bonding of glycerol as evidenced by calculated heat of vaporization being in excellent agreement with experiment. Computed condensed phase data, including crystal lattice parameters and volumes and densities of aqueous solutions are in better agreement with experimental data as compared to the corresponding additive model. Such improvements are anticipated to significantly improve the treatment of polymers in general, including biological macromolecules.

Peptidyl 3-hydroxyproline binding properties of type I collagen suggest a function in fibril supramolecular assembly

Proline residues in collagens are extensively hydroxylated post-translationally. A rare form of this modification, (3S,2S)-l-hydroxyproline (3Hyp), remains without a clear function. Disruption of the enzyme complex responsible for prolyl 3-hydroxylation results in severe forms of recessive osteogenesis imperfecta (OI). These OI types exhibit a loss of or reduction in the level of 3-hydroxylation at two proline residues, α1(I) Pro986 and α2(I) Pro707. Whether the resulting brittle bone phenotype is caused by the lack of the 3-hydroxyl addition or by another function of the enzyme complex is unknown. We have speculated that the most efficient mechanism for explaining the chemistry of collagen intermolecular cross-linking is for pairs of collagen molecules in register to be the subunit that assembles into fibrils. In this concept, the exposed hydroxyls from 3Hyp are positioned within mutually interactive binding motifs on adjacent collagen molecules that contribute through hydrogen bonding to the process of fibril supramolecular assembly. Here we report observations on the physical binding properties of 3Hyp in collagen chains from experiments designed to explore the potential for interaction using synthetic collagen-like peptides containing 3Hyp. Evidence of self-association was observed between a synthetic peptide containing 3Hyp and the CB6 domain of the α1(I) chain, which contains the single fully 3-hydroxylated proline. Using collagen from a case of severe recessive OI with a CRTAP defect, in which Pro986 was minimally 3-hydroxylated, such binding was not observed. Further study of the role of 3Hyp in supramolecular assembly is warranted for understanding the evolution of tissue-specific variations in collagen fibril organization.

Effects of phosphate-buffered saline concentration and incubation time on the mechanical and structural properties of electrochemically aligned collagen threads

A key step during the synthesis of collagen constructs is the incubation of monomeric collagen in phosphate buffer saline (PBS) to promote fibrillogenesis in the collagen network. Optimal PBS-treatment conditions for monomeric collagen solutions to induce gelation are well established in the literature. Recently, a report in the literature (Cheng et al 2008 Biomaterials 29 3278-88) showed a novel method to fabricate highly oriented electrochemically aligned collagen (ELAC) threads which have orders of magnitude greater packing density than collagen gels. The optimal PBS-treatment conditions for induction of D-banding pattern in such a dense and anisotropic collagen network are unknown. This study aimed to optimize PBS treatment of ELAC threads by investigating the effect of phosphate ion concentration (0.5×, 1×, 5× and 10×) and incubation time (3, 12 and 96 h) on the mechanical strength and ultrastructural organization by monotonic mechanical testing, small angle x-ray scattering and transmission electron microscopy (TEM). ELAC threads incubated in water (no PBS) served as the control. ELAC threads incubated in 1× PBS showed significantly higher extensibility compared to those in 0.5× or 10× PBS along with the presence of D-banded patterns with a periodicity of 63.83 nm. Incubation of ELAC threads in 1× PBS for 96 h resulted in significantly higher ultimate stress compared to 3 or 12 h. However, these threads lacked the D-banding pattern. TEM observations showed no significant differences in the microfibril diameter distribution of ELAC threads treated with or without PBS. This indicates that microfibrils lacked D-banding following electrochemical alignment and the subsequent PBS-treatment-induced D-banding by reorganization within microfibrils. It was concluded that incubation of aligned collagen in 1× PBS for 12 h results in mechanically competent, D-banded ELAC threads which can be used for the regeneration of load bearing tissues such as tendons and ligaments.

A molecular dynamics study of the interprotein interactions in collagen fibrils

Molecular dynamics simulations of collagen are used to investigate at the atomistic level the nature of the interprotein interactions that are present within a collagen fibril, and which are responsible for the fibril's thermodynamic stability. Simulations both of a collagen fibril and of a fully solvated tropcollagen are compared in order to study the interactions that arise between the proteins upon the process of fibrillogenesis. The interactions studied include direct interprotein hydrogen bonds, water-mediated interprotein hydrogen bonds, and hydrophobic interactions. The simulations are used to quantify the number of interprotein interactions that form; to study which functional groups contribute most towards the interactions; and to study the spatial distribution of interprotein interactions throughout the fibril's D period. The processes of collagen fibrillogenesis and protein folding are then compared with each other, because these two physical processes share many similarities in concept, and the latter has been more widely studied. Molecular dynamics simulations of a bacteriophage T4 lysozyme protein, both in its native state and in and unfolded state, are used as an illustrative example of a typical protein folding process, for direct comparison with the collagen simulations.

Photochemical crosslinking of soluble wool keratins produces a mechanically stable biomaterial that supports cell adhesion and proliferation

Keratins extracted from various "hard tissues" such as wool, hair, and nails are increasingly being investigated as a source of abundant, biocompatible materials. In this study we explored a recent photochemical method to crosslink solubilized wool keratoses, with the aim of producing a mechanically favorable biomaterial. Wool proteins were isolated by oxidizing the disulfides and extracting the resulting soluble keratoses. The α- and γ-keratose fractions were analyzed by liquid chromatography-mass spectrometry to identify their constituent proteins. Hydrogels were produced by covalent crosslinking of the α-keratoses via a photo-oxidative process catalyzed by blue light, a ruthenium complex, and persulfate. The presence of dityrosine crosslinks was demonstrated by high performance liquid chromatography and mass spectrometry analyses. The crosslinked α-keratose material had moderate tensile strength and elasticity, and high adhesive strength. The material displayed modest shrinking after crosslinking, however the shrinking could be prevented by crosslinking in the presence of 2.5% glycerol, resulting in gels that did not shrink or swell. Small solutes such as Tris and glycerol influenced the crosslink density and elastic modulus of the crosslinked material. The α-keratose was able to support adhesion and growth of NIH/3T3 fibroblasts in vitro. The fabrication of mechanically stable keratin biomaterials by this facile photo-crosslinking method may be useful for various tissue engineering applications.

Molecular basis for optical clearing of collagenous tissues

Molecular interactions of optical clearing agents were investigated using a combination of molecular dynamics (MD) simulations and optical spectroscopy. For a series of sugar alcohols with low to high optical clearing potential, Raman spectroscopy and integrating sphere measurements were used to quantitatively characterize tissue water loss and reduction in light scattering following agent exposures. The rate of tissue water loss was found to correlate with agent optical clearing potential, but equivalent tissue optical clearing was measured in native and fixed tissue in vitro, given long-enough exposure times to the polyol series. MD simulations showed that the rate of tissue optical clearing correlated with the preferential formation of hydrogen bond bridges between agent and collagen. Hydrogen bond bridge formation disrupts the collagen hydration layer and facilitates replacement by a chemical agent to homogenize tissue refractive index. However, the reduction in tissue light scattering did not correlate with the agent index of refraction. Our results suggest that a necessary property of optical clearing agents is hyperosmolarity to tissue, but that the most effective agents with the highest rates of optical clearing are a subset with the highest collagen solubilities.

Synchrotron X-ray diffraction study of load partitioning during elastic deformation of bovine dentin

The elastic properties of dentin, a biological composite consisting of stiff hydroxyapatite (HAP) nano-platelets within a compliant collagen matrix, are determined by the volume fraction of these two phases and the load transfer between them. We have measured the elastic strains in situ within the HAP phase of bovine dentine by high energy X-ray diffraction for a series of static compressive stresses at ambient temperature. The apparent HAP elastic modulus (ratio of applied stress to elastic HAP strain) was found to be 18+/-2GPa. This value is significantly lower than the value of 44GPa predicted by the lower bound load transfer Voigt model, using HAP and collagen volume fractions determined by thermo-gravimetric analysis. This discrepancy is explained by (i) a reduction in the intrinsic Young's modulus of the nano-size HAP platelets due to the high fraction of interfacial volume and (ii) an increase in local stresses due to stress concentration around the dentin tubules.

Stability of the hydration layer of tropocollagen: A QM study

Collagen is a triple helical protein, highly hydrated in nature. Bella and Berman (J Mol Biol 1996, 264, 734) have reported the structure of the first hydration layer. Water molecules form bridges of different length around the POG repeats and self assemble into left-handed helical water threads. To explore the stability of these specifically hydrated places, we have designed suitable QM models: each comprises a triple helix formed by 18 residues surrounded by 8 to 12 explicit waters. Two sets of amino acids were used, one standing for the core structural subunit of tropocollagen (POG-model) and one for its natural enzyme recognition sites (AAG-model). We have determined the stability order of the water binding places, the strongest being -8.1 kcal mol(-1), while the weakest -6.1 kcal mol(-1) per hydrogen bond. In X-ray structures, each triplet of tropocollagen is shielded by six to nine water molecules. Beside the mandatory six, the "surplus" three water molecules further strengthen the binding of all the others. However, the displacement of selected water molecules turns out to be energy neutral. These water binding places on the surface of the triple helix can provide explanation on how an almost liquid-like hydration environment exists between the closely packed tropocollagens (Henkelman et al., Magn Reson Med 1994, 32, 592). It seems that these water reservoirs or buffers can provide space for "hole conduction" of water molecules and thus contribute to the elasticity of collagen.

Revisiting optical clearing with dimethyl sulfoxide (DMSO)

Functional optical characterization of disease progression and response to therapy suffers from loss of spatial resolution and imaging depth due to scattering. Here we report on the ability of dimethyl sulfoxide (DMSO) alone to reduce the optical scattering of skin. We observed a threefold reduction in the scattering of skin with topical DMSO application. With an in vivo window chamber model, we observed a threefold increase in light transmittance through the preparation and enhanced visualization of subsurface microvasculature. Collectively, our data demonstrate the potential of DMSO alone to mitigate effects of scattering, which we expect will improve molecular imaging studies.

Triple-helical peptides: an approach to collagen conformation, stability, and self-association

Peptides have been an integral part of the collagen triple-helix structure story, and have continued to serve as useful models for biophysical studies and for establishing biologically important sequence-structure-function relationships. High resolution structures of triple-helical peptides have confirmed the basic Ramachandran triple-helix model and provided new insights into the hydration, hydrogen bonding, and sequence dependent helical parameters in collagen. The dependence of collagen triple-helix stability on the residues in its (Gly-X-Y)(n) repeating sequence has been investigated by measuring melting temperatures of host-guest peptides and an on-line collagen stability calculator is now available. Although the presence of Gly as every third residue is essential for an undistorted structure, interruptions in the repeating (Gly-X-Y)(n) amino acid sequence pattern are found in the triple-helical domains of all nonfibrillar collagens, and are likely to play a role in collagen binding and degradation. Peptide models indicate that small interruptions can be incorporated into a rod-like triple-helix with a highly localized effect, which perturbs hydrogen bonds and places the standard triple-helices on both ends out of register. In contrast to natural interruptions, missense mutations which replace one Gly in a triple-helix domain by a larger residue have pathological consequences, and studies on peptides containing such Gly substitutions clarify their effect on conformation, stability, and folding. Recent studies suggest peptides may also be useful in defining the basic principles of collagen self-association to the supramolecular structures found in tissues.

Bonding of macromolecular hydrogels using perturbants

This work describes a method to bond patterned macromolecular gels into monolithic structures using perturbants. Bonding strengths for a variety of solutes follow a Hofmeister ordering; this result and optical measurements indicate that bonding occurs by reversible perturbation of contacting gels. The resulting microfluidic gels are mechanically robust and can serve as scaffolds for cell culture.

Effect of oxazolidine E on collagen fibril formation and stabilization of the collagen matrix

Oxazolidine E, an aldehydic cross-linking agent, is used to impart hydrothermal stability to collagen. The purpose of this study was to investigate the exact nature of oxazolidine E induced cross-links with collagen by using synthetic peptides having sequence homology with collagen type I. Tandem mass spectrometry revealed the formation of methylol and Schiff-base adducts upon reaction of oxazolidine E with the peptides. This was confirmed by allowing the reaction to proceed under reducing conditions using cyanoborohydride. Mass spectrometry (MS)-MS analysis clearly showed interaction of tryptophan and lysine residues with oxazolidine E and demonstrated that arginine could be cross-linked with glycine in the presence of oxazolidine E through the formation of a methylene bridge. Collagen fibrils regenerated from monomers in the presence and absence of oxazolidine E were studied using atomic force microscopy to investigate morphological alterations. Regenerated fibrils showing the typical 65 nm D-banding pattern were obtained from those formed both in the presence and absence of oxazolidine E, and there was no evidence of a change in the D-periodicity of these fibrils. This indicated that oxazolidine E does not hinder collagen molecules from correctly aligning to form the quarter-stagger structure.

Correlation between collagen solubility and skin optical clearing using sugars

BACKGROUND AND OBJECTIVE

Light scattering from collagen within skin limits light-based therapeutics while increasing the risk of epidermal thermal injury. Specific chemicals show the ability to reduce light scattering by reversibly altering the optical properties of skin. This study examines the correlation between collagen solubility and the optical clearing potential (OCP) of sugars and sugar-alcohols using in vitro rodent skin.

MATERIALS AND METHODS

Collagen solubility in dextrose, fructose, sucrose, and sorbitol was measured using near-UV spectroscopy. Light transmittance, reflectance, and rodent skin thickness were measured (giving skin reduced scattering coefficient) before and after exposure of the dermal surface to sugars and sugar-alcohols. OCP was calculated as the ratio of reduced scattering coefficients before and after exposures.

RESULTS

Dextrose, fructose, sucrose, and sorbitol had at least twice the collagen solubility and twice the OCP as compared to glycerol. In general, collagen solubility correlated with each agent's ability to optically clear rodent skin.

CONCLUSION

These results demonstrate that sugar and sugar-alcohol interaction with collagen are a primary event in tissue optical clearing.

Effect of the Structural Water on the Mechanical Properties of Collagen-like Microfibrils: A Molecular Dynamics Study

The objective of this paper is to investigate the role played by the structural water on the intermolecular sliding between collagen-like 1QSU peptides in a microfibril under deformation. Three modes of deformation are used to generate intermolecular sliding: forced axial stretching (case I) or sliding (case II) of a central peptide monomer (while other surrounding monomers are fixed); and cantilever bending (case III) under a terminal lateral load. The force-displacement curve of each deformation mode is derived using a module called Steered Molecular Dynamics (SMD) in a molecular dynamics package NAMD under the CHARMM22 force field. Each calculation is carried out twice, one in the presence of structural water, one without. It is found that the structural water is a weak "lubricant" in forced axial stretching (case I), but it functions as a "glue" in forced axial sliding (case II) and cantilever bending (case III). A change in the pulling speed does not significantly alter the force-displacement behavior in axial stretching (case I) and sliding (case II), but it does in cantilever bending (case III). The additional resistance contributed by the structural water is attributed to the additional energy cost in breaking the water-mediated hydrogen bonds (water bridges).

Interactions of Collagen Types I and II with Chondroitin Sulfates A−C and Their Effect on Osteoblast Adhesion

Collagen has found use as a scaffold material for tissue engineering as well as a coating material for implants. The main aim of this study was to compare the ability of the collagen types I and II to bind preparations of the chondroitin sulfate types A-C (CS A, CS B, CS C). In addition, the effect of the three CS preparations on the extent of collagen incorporated into fibrils and the morphology of collagen fibrils was investigated, as was the influence of collagen fibril coatings containing CS A-C on titanium surfaces on the adhesion of primary rat osteoblasts. Fibrils of both collagen types bound a higher mass of CS C than CS B and a greater mass of CS B than CS A per milligram of fibrils formed. Fibrils of collagen type II bound a higher mass of CS B and C than collagen I fibrils. The proportion of collagen incorporated into fibrils decreased with increasing CS A and CS C concentration but not with increasing CS B concentration. All three CS preparations caused collagen I and II fibrils to become thinner. CS A and CS B but not CS C appeared to stimulate the formation of focal adhesions by osteoblasts after incubation for 2 hours. These results could be of importance when selecting collagen type or CS type as materials for implant coatings or tissue engineering scaffolds.