Self-assembly and elasticity of hierarchical proteoglycan–hyaluronan brushes††Electronic supplementary information (ESI) available: Variations in areal mass density upon SLB and SAv monolayer formation determined by SE (Fig. S1). See DOI: 10.1039/c3sm51213d

Sweet but Challenging: Tackling the Complexity of GAGs with Engineered Tailor-Made Biomaterials

Glycosaminoglycans (GAGs) play a crucial role in tissue homeostasis by regulating the activity and diffusion of bioactive molecules. Incorporating GAGs into biomaterials has emerged as a widely adopted strategy in medical applications, owing to their biocompatibility and ability to control the release of bioactive molecules. Nevertheless, immobilized GAGs on biomaterials can elicit distinct cellular responses compared to their soluble forms, underscoring the need to understand the interactions between GAG and bioactive molecules within engineered functional biomaterials. By controlling critical parameters such as GAG type, density, and sulfation, it becomes possible to precisely delineate GAG functions within a biomaterial context and to better mimic specific tissue properties, enabling tailored design of GAG-based biomaterials for specific medical applications. However, this requires access to pure and well-characterized GAG compounds, which remains challenging. This review focuses on different strategies for producing well-defined GAGs and explores high-throughput approaches employed to investigate GAG-growth factor interactions and to quantify cellular responses on GAG-based biomaterials. These automated methods hold considerable promise for improving the understanding of the diverse functions of GAGs. In perspective, the scientific community is encouraged to adopt a rational approach in designing GAG-based biomaterials, taking into account the in vivo properties of the targeted tissue for medical applications.

V3: an enigmatic isoform of the proteoglycan versican

V3 is an isoform of the extracellular matrix (ECM) proteoglycan (PG) versican generated through alternative splicing of the versican gene such that the two major exons coding for sequences in the protein core that support chondroitin sulfate (CS) glycosaminoglycan (GAG) chain attachment are excluded. Thus, versican V3 isoform carries no GAGs. A survey of PubMed reveals only 50 publications specifically on V3 versican, so it is a very understudied member of the versican family, partly because to date there are no antibodies that can distinguish V3 from the CS-carrying isoforms of versican, that is, to facilitate functional and mechanistic studies. However, a number of in vitro and in vivo studies have identified the expression of the V3 transcript during different phases of development and in disease, and selective overexpression of V3 has shown dramatic phenotypic effects in "gain and loss of function" studies in experimental models. Thus, we thought it would be useful and instructive to discuss the discovery, characterization, and the putative biological importance of the enigmatic V3 isoform of versican.

A bio-inspired nano-material recapitulating the composition, ultra-structure, and function of the glycosaminoglycan-rich extracellular matrix of nucleus pulposus

The nucleus pulposus (NP) of intervertebral disc represents a soft gel consisting of glycosaminoglycans (GAGs)-rich extracellular matrix (ECM). Significant loss of GAGs and normal functions are the most prevalent changes in degenerated disc. Attempts targeted to incorporate GAGs into collagen fibrous matrices have been made but the efficiency is very low, and the resulting structures showed no similarity with native NP. Inspired by the characteristic composition and structures of the ECM of native NP, here, we hypothesize that by chemically modifying the collagen (Col) and hyaluronic acid (HA) and co-precipitating with GAGs, a bio-inspired nano-material recapitulating the composition, ultra-structure and function of the GAG-rich ECM will be fabricated. Compositionally, the bio-inspired nano-material namely Aminated Collagen-Aminated Hyaluronic Acid-GAG (aCol-aHA-GAG) shows a record high GAG/hydroxyproline ratio up to 39.1:1 in a controllable manner, out-performing that of the native NP. Ultra-structurally, the nano-material recapitulates the characteristic 'nano-beads' (25 nm) and 'bottle-brushes' (133 nm) features as those found in native NP. Functionally, the nano-material supports the viability and maintains the morphological and phenotypic markers of bovine NP cells, and shows comparable mechanical properties of native NP. This work contributes to the development of a compositionally, structurally, and functionally biomimetic nano-material for NP tissue engineering.

Presence of chondroitin sulphate and requirement for heparan sulphate biosynthesis in the developing zebrafish inner ear

Epithelial morphogenesis to form the semicircular canal ducts of the zebrafish inner ear depends on the production of the large glycosaminoglycan hyaluronan, which is thought to contribute to the driving force that pushes projections of epithelium into the lumen of the otic vesicle. Proteoglycans are also implicated in otic morphogenesis: several of the genes coding for proteoglycan core proteins, together with enzymes that synthesise and modify their polysaccharide chains, are expressed in the developing zebrafish inner ear. In this study, we demonstrate the highly specific localisation of chondroitin sulphate to the sites of epithelial projection outgrowth in the ear, present before any morphological deformation of the epithelium. Staining for chondroitin sulphate is also present in the otolithic membrane, whereas the otoliths are strongly positive for keratan sulphate. We show that heparan sulphate biosynthesis is critical for normal epithelial projection outgrowth, otolith growth and tethering. In the ext2 mutant ear, which has reduced heparan sulphate levels, but continues to produce hyaluronan, epithelial projections are rudimentary, and do not grow sufficiently to meet and fuse to form the pillars of tissue that normally span the otic lumen. Staining for chondroitin sulphate and expression of versican b, a chondroitin sulphate proteoglycan core protein gene, persist abnormally at high levels in the unfused projections of the ext2 mutant ear. We propose a model for wild-type epithelial projection outgrowth in which hyaluronan and proteoglycans are linked to form a hydrated gel that fills the projection core, with both classes of molecule playing essential roles in zebrafish semicircular canal morphogenesis.

A quartz crystal microbalance method to quantify the size of hyaluronan and other glycosaminoglycans on surfaces

Hyaluronan (HA) is a major component of peri- and extra-cellular matrices and plays important roles in many biological processes such as cell adhesion, proliferation and migration. The abundance, size distribution and presentation of HA dictate its biological effects and are also useful indicators of pathologies and disease progression. Methods to assess the molecular mass of free-floating HA and other glycosaminoglycans (GAGs) are well established. In many biological and technological settings, however, GAGs are displayed on surfaces, and methods to obtain the size of surface-attached GAGs are lacking. Here, we present a method to size HA that is end-attached to surfaces. The method is based on the quartz crystal microbalance with dissipation monitoring (QCM-D) and exploits that the softness and thickness of films of grafted HA increase with HA size. These two quantities are sensitively reflected by the ratio of the dissipation shift (ΔD) and the negative frequency shift (- Δf) measured by QCM-D upon the formation of HA films. Using a series of size-defined HA preparations, ranging in size from ~ 2 kDa tetrasaccharides to ~ 1 MDa polysaccharides, we establish a monotonic yet non-linear standard curve of the ΔD/ - Δf ratio as a function of HA size, which reflects the distinct conformations adopted by grafted HA chains depending on their size and surface coverage. We demonstrate that the standard curve can be used to determine the mean size of HA, as well as other GAGs, such as chondroitin sulfate and heparan sulfate, of preparations of previously unknown size in the range from 1 to 500 kDa, with a resolution of better than 10%. For polydisperse samples, our analysis shows that the process of surface-grafting preferentially selects smaller GAG chains, and thus reduces the average size of GAGs that are immobilised on surfaces comparative to the original solution sample. Our results establish a quantitative method to size HA and other GAGs grafted on surfaces, and also highlight the importance of sizing GAGs directly on surfaces. The method should be useful for the development and quality control of GAG-based surface coatings in a wide range of research areas, from molecular interaction analysis to biomaterials coatings.

Single-Molecule Stretching Shows Glycosylation Sets Tension in the Hyaluronan-Aggrecan Bottlebrush

Large bottlebrush complexes formed from the polysaccharide hyaluronan (HA) and the proteoglycan aggrecan contribute to cartilage compression resistance and are necessary for healthy joint function. A variety of mechanical forces act on these complexes in the cartilage extracellular matrix, motivating the need for a quantitative description that links their structure and mechanical response. Studies using electron microscopy have imaged the HA-aggrecan brush but require adsorption to a surface, dramatically altering the complex from its native conformation. We use magnetic tweezers force spectroscopy to measure changes in extension and mechanical response of an HA chain as aggrecan monomers bind and form a bottlebrush. This technique directly measures changes undergone by a single complex with time and under varying solution conditions. Upon addition of aggrecan, we find a large swelling effect manifests when the HA chain is under very low external tension (i.e., stretching forces less than ∼1 pN). We use models of force-extension behavior to show that repulsion between the aggrecans induces an internal tension in the HA chain. Through reference to theories of bottlebrush polymer behavior, we demonstrate that the experimental values of internal tension are consistent with a polydisperse aggrecan population, likely caused by varying degrees of glycosylation. By enzymatically deglycosylating the aggrecan, we show that aggrecan glycosylation is the structural feature that causes HA stiffening. We then construct a simple stochastic binding model to show that variable glycosylation leads to a wide distribution of internal tensions in HA, causing variations in the mechanics at much longer length scales. Our results provide a mechanistic picture of how flexibility and size of HA and aggrecan lead to the brush architecture and mechanical properties of this important component of cartilage.

Effect of calcium ions and pH on the morphology and mechanical properties of hyaluronan brushes

Hyaluronan (HA) is a linear, regular polysaccharide that plays as a chief structural and functional component in peri- and extracellular matrices, thus contributing significantly to many basic cellular processes. To understand more comprehensively the response of the supramolecular organization of HA polymers to changes in their aqueous environment, we study the effects of Ca2+ concentration and pH on the morphology and rigidity of films of end-grafted HA polymers on planar supports (HA brushes), as a well-defined in vitro model system of HA-rich matrices, by reflection interference contrast microscopy and quartz crystal microbalance. The thickness and softness of HA brushes decrease significantly with Ca2+ concentration but do not change with pH, within the physiological ranges of these parameters. The effect of Ca2+ on HA brush thickness is virtually identical to the effect of Na+ at 10-fold higher concentrations. Moreover, the thickness and softness of HA brushes decrease appreciably upon HA protonation at pH less than 6. Effects of pH and calcium ions are fully reversible over large parameter ranges. These findings are relevant for understanding the supramolecular organization and dynamics of HA-rich matrices in biological systems and will also benefit the rational design of synthetic HA-rich materials with tailored properties.

Investigation of the lubrication properties and synergistic interaction of biocompatible liposome-polymer complexes applicable to artificial joints

Achievement of efficient biolubrication is essential for the design of artificial joints with long lifetimes. This study examines the frictional behaviors and adsorption structures of liposomes and liposome complexes with biocompatible polymers to reveal the underlying lubrication mechanisms between biomimetic bearing surfaces of polyetheretherketone (PEEK) and silicon nitride (Si₃N₄). The liposomes with increasing carbon chain lengths exhibit the remarkable lubrication capabilities that correlate strongly with the structural integrity of small unilamellar vesicles adsorbed on the Si₃N₄ surfaces, while the bilayer structures weaken the stability of vesicles against rupture and cause the increase of friction. The synergistic interaction of liposomes and biocompatible negative-charged polymer leads to the formation of a boundary-lubricating layer with high-density liposome-polymer complex structures that can efficiently improve the lubrication properties of liposomes. Our findings might have implications for future biolubrication investigations on biocompatible liposome-polymer complexes applicable to artificial joints at the specified macroscale conditions.

Elastohydrodynamic Lift at a Soft Wall

We study experimentally the motion of nondeformable microbeads in a linear shear flow close to a wall bearing a thin and soft polymer layer. Combining microfluidics and 3D optical tracking, we demonstrate that the steady-state bead-to-surface distance increases with the flow strength. Moreover, such lift is shown to result from flow-induced deformations of the layer, in quantitative agreement with theoretical predictions from elastohydrodynamics. This study thus provides the first experimental evidence of "soft lubrication" at play at small scale, in a system relevant, for example, to the physics of blood microcirculation.

Glycosaminoglycans in extracellular matrix organisation: are concepts from soft matter physics key to understanding the formation of perineuronal nets?

Conventional wisdom has it that proteins fold and assemble into definite structures, and that this defines their function. Glycosaminoglycans (GAGs) are different. In most cases the structures they form have a low degree of order, even when interacting with proteins. Here, we discuss how physical features common to all GAGs-hydrophilicity, charge, linearity and semi-flexibility-underpin the overall properties of GAG-rich matrices. By integrating soft matter physics concepts (e.g. polymer brushes and phase separation) with our molecular understanding of GAG-protein interactions, we can better comprehend how GAG-rich matrices assemble, what their properties are, and how they function. Taking perineuronal nets (PNNs)-a GAG-rich matrix enveloping neurons-as a relevant example, we propose that microphase separation determines the holey PNN anatomy that is pivotal to PNN functions.

Single-Molecule Imaging of Proteoglycans in the Pericellular Matrix

The pericellular matrix is a robust, hyaluronan-rich polymer brush-like structure that controls access to the cell surface, and plays an important role in cell adhesion, migration, and proliferation. We report the observation of single bottlebrush proteoglycan dynamics in the pericellular matrix of living chondrocytes. Our investigations show that the pericellular matrix undergoes gross extension on the addition of exogenous aggrecan, and that this extension is significantly in excess of that observed in traditional particle exclusion assays. The mean-square displacement of single, bound proteoglycans increases with distance to cell surface, indicating reduced confinement by neighboring hyaluronan-aggrecan complexes. This is consistent with published data from quantitative particle exclusion assays that show openings in the pericellular matrix microstructure ranging from ∼150 nm near the cell surface to ∼400 nm near the cell edge. In addition, the mobility of tethered aggrecan drops significantly when the cell coat is enriched with bottlebrush proteoglycans. Single-molecule imaging in this thick polysaccharide matrix on living cells has significant promise in the drive to elucidate the role of the pericellular coat in human health.

Cell Surface Access Is Modulated by Tethered Bottlebrush Proteoglycans

The hyaluronan-rich pericellular matrix (PCM) plays physical and chemical roles in biological processes ranging from brain plasticity, to adhesion-dependent phenomena such as cell migration, to the onset of cancer. This study investigates how the spatial distribution of the large negatively charged bottlebrush proteoglycan, aggrecan, impacts PCM morphology and cell surface access. The highly localized pericellular milieu limits transport of nanoparticles in a size-dependent fashion and sequesters positively charged molecules on the highly sulfated side chains of aggrecan. Both rat chondrocyte and human mesenchymal stem cell PCMs possess many unused binding sites for aggrecan, showing a 2.5x increase in PCM thickness from ∼7 to ∼18 μm when provided exogenous aggrecan. Yet, full extension of the PCM occurs well below aggrecan saturation. Hence, cells equipped with hyaluronan-rich PCM can in principle manipulate surface accessibility or sequestration of molecules by tuning the bottlebrush proteoglycan content to alter PCM porosity and the number of electrostatic binding sites.

Insight into the Tribological Behavior of Liposomes in Artificial Joints

Liposomes are widely used in drug delivery and gene therapy, and their new role as boundary lubricant in natural/artificial joints has been found in recent years. In this study, the tribological properties of liposomes on titanium alloy (Ti6Al4 V)/UHMWPE interface were studied by a ball-on-disc tribometer. The efficient reduction of friction coefficient and wear on both surfaces under various velocities and loads is found. A multilayer structure of physically adsorbed liposomes on Ti6Al4 V surface was also observed by atomic force microscope (AFM). Except for the hydration mechanism by phosphatidylcholine (PC) groups, the well-performed tribological properties by liposomes is also attributed to the existence of adsorbed liposome layers on both surfaces, which could reduce asperities contact and show great bearing capacity. This work enriches the research on liposomes for lubrication improvement on artificial surface and shows their value in clinical application.

Micromechanical Analysis of the Hyaluronan-Rich Matrix Surrounding the Oocyte Reveals a Uniquely Soft and Elastic Composition

The cumulus cell-oocyte complex (COC) matrix is an extended coat that forms around the oocyte a few hours before ovulation and plays vital roles in oocyte biology. Here, we analyzed the micromechanical response of mouse COC matrix by colloidal-probe atomic force microscopy. We found that the COC matrix is elastic insofar as it does not flow and its original shape is restored after force release. At the same time, the COC matrix is extremely soft. Specifically, the most compliant parts of in vivo and in vitro expanded COC matrices yielded Young's modulus values of 0.5 ± 0.1 Pa and 1.6 ± 0.3 Pa, respectively, suggesting both high porosity and a large mesh size (≥100 nm). In addition, the elastic modulus increased progressively with indentation. Furthermore, using optical microscopy to correlate these mechanical properties with ultrastructure, we discovered that the COC is surrounded by a thick matrix shell that is essentially devoid of cumulus cells and is enhanced upon COC expansion in vivo. We propose that the pronounced nonlinear elastic behavior of the COC matrix is a consequence of structural heterogeneity and serves important functions in biological processes such as oocyte transport in the oviduct and sperm penetration.

Controlled Immobilization Strategies to Probe Short Hyaluronan-Protein Interactions

Well-controlled grafting of small hyaluronan oligosaccharides (sHA) enables novel approaches to investigate biological processes such as angiogenesis, immune reactions and cancer metastasis. We develop two strategies for covalent attachment of sHA, a fast high-density adsorption and a two-layer system that allows tuning the density and mode of immobilization. We monitored the sHA adlayer formation and subsequent macromolecular interactions by label-free quartz crystal microbalance with dissipation (QCM-D). The modified surfaces are inert to unspecific protein adsorption, and yet retain the specific binding capacity of sHA. Thus they are an ideal tool to study the interactions of hyaluronan-binding proteins and short hyaluronan molecules as demonstrated by the specific recognition of LYVE-1 and aggrecan. Both hyaladherins recognize sHA and the binding is independent to the presence of the reducing end.

Interaction of Hyaluronan with Cationic Nanoparticles

The polysaccharide hyaluronan (HA) is a main component of peri- and extracellular matrix, and an attractive molecule for materials design in tissue engineering and nanomedicine. Here, we study the morphology of complexes that form upon interaction of nanometer-sized amine-coated gold particles with this anionic, linear, and regular biopolymer in solution and grafted to a surface. We find that cationic nanoparticles (NPs) have profound effects on HA morphology on the molecular and supramolecular scale. Quartz crystal microbalance (QCM-D) shows that depending on their relative abundance, cationic NPs promote either strong compaction or swelling of films of surface-grafted HA polymers (HA brushes). Transmission electron and atomic force microscopy reveal that the NPs do also give rise to complexes of distinct morphologies-compact nanoscopic spheres and extended microscopic fibers-upon interaction with HA polymers in solution. In particular, stable and hydrated spherical complexes of single HA polymers with NPs can be prepared when balancing the ionizable groups on HA and NPs. The observed self-assembly phenomena could be useful for the design of drug delivery vehicles and a better understanding of the reorganization of HA-rich synthetic or biological matrices.

Supramolecular synergy in the boundary lubrication of synovial joints

Hyaluronan, lubricin and phospholipids, molecules ubiquitous in synovial joints, such as hips and knees, have separately been invoked as the lubricants responsible for the remarkable lubrication of articular cartilage; but alone, these molecules cannot explain the extremely low friction at the high pressures of such joints. We find that surface-anchored hyaluronan molecules complex synergistically with phosphatidylcholine lipids present in joints to form a boundary lubricating layer, which, with coefficient of friction μ≈0.001 at pressures to over 100 atm, has a frictional behaviour resembling that of articular cartilage in the major joints. Our findings point to a scenario where each of the molecules has a different role but must act together with the others: hyaluronan, anchored at the outer surface of articular cartilage by lubricin molecules, complexes with joint phosphatidylcholines to provide the extreme lubrication of synovial joints via the hydration–lubrication mechanism.

Interacting cartilage surfaces in synovial joints experience very low levels of friction, allowing unhindered mechanical motion. Here, the authors propose how a synergistic interaction of hyaluronan, lubricin and phospholipids, molecules that are ubiquitous in joints, may lead to this high level of lubrication.