Molecular conformations and dynamics in the extracellular matrix of mammalian structural tissues: Solid-state NMR spectroscopy approaches

On the role of the glycosylation of type I collagen in bone

Glycan-protein interactions play a crucial role in biology, providing additional functions capable of inducing biochemical and cellular responses. In the extracellular matrix of bone, this type of interactions is ubiquitous. During the synthesis of the collagen molecule, glycans are post-translationally added to specific lysine residues through an enzymatically catalysed hydroxylation and subsequent glycosylation. During and after fibril assembly, proteoglycans are essential for maintaining tissue structure, porosity, and integrity. Glycosaminoglycans (GAGs), the carbohydrate chains attached to interstitial proteoglycans, are known to be involved in mineralization. They can attract and retain water, which is critical for the mechanical properties of bone. In addition, like other long-lived proteins, collagen is susceptible to glycation. Prolonged exposure of the amine group to glucose eventually leads to the formation of advanced glycation end-products (AGEs). Changes in the degree of glycosylation and glycation have been identified in bone pathologies such as osteogenesis imperfecta and diabetes and appear to be associated with a reduction in bone quality. However, how these changes affect mineralization is not well understood. Based on the literature review, we hypothesize that the covalently attached carbohydrates may have a water-attracting function similar to that of GAGs, but at different lengths and timescales in the bone formation process. Glycosylation potentially increases the hydration around the collagen triple helix, leading to increased mineralization (hypermineralization) after water has been replaced by mineral. Meanwhile, glycation leads to the formation of crosslinking AGEs, which are associated with a decrease in hydration levels, reducing the mechanical properties of bone.

Unveiling Charge-Pair Salt-Bridge Interaction Between GAGs and Collagen Protein in Cartilage: Atomic Evidence from DNP-Enhanced ssNMR at Natural Isotopic Abundance

The interactions between glycosaminoglycans (GAGs) and proteins are essential in numerous biochemical processes that involve ion-pair interactions. However, there is no evidence of direct and specific interactions between GAGs and collagen proteins in native cartilage. The resolution of solid-state NMR (ssNMR) can offer such information but the detection of GAG interactions in cartilage is limited by the sensitivity of the experiments when 13C and 15N isotopes are at natural abundance. In this communication, this limitation is overcome by taking advantage of dynamic nuclear polarization (DNP)-enhanced magic-angle spinning (MAS) experiments to obtain two-dimensional (2D) 15N-13C and 13C-13C correlations on native samples at natural abundance. These experiments unveiled inter-residue correlations in the aliphatic regions of the collagen protein previously unobserved. Additionally, our findings provide direct evidence of charge-pair salt-bridge interactions between negatively charged GAGs and positively charged arginine (Arg) residues of collagen protein. We also identified potential hydrogen bonding interactions between hydroxyproline (Hyp) and GAGs, offering atomic insights into the biochemical interactions within the extracellular matrix of native cartilage. Our approach may provide a new avenue for the structural characterization of other native systems.

Resolving Atomic-Level Dynamics and Interactions of High-Molecular-Weight Hyaluronic Acid by Multidimensional Solid-State NMR

High-molecular-weight (HMW) hyaluronic acid (HA) is a highly abundant natural polysaccharide and a fundamental component of the extracellular matrix (ECM). Its size and concentration regulate tissues' macro- and microenvironments, and its upregulation is a hallmark feature of certain tumors. Yet, the conformational dynamics of HMW-HA and how it engages with the components of the ECM microenvironment remain poorly understood at the molecular level. Probing the molecular structure and dynamics of HMW polysaccharides in a hydrated, physiological-like environment is crucial and also technically challenging. Here, we deploy advanced magic-angle spinning (MAS) solid-state NMR spectroscopy in combination with isotopic enrichment to enable an in-depth study of HMW-HA to address this challenge. This approach resolves multiple coexisting HA conformations and dynamics as a function of environmental conditions. By combining 13C-labeled HA with unlabeled ECM components, we detect by MAS NMR HA-specific changes in global and local conformational dynamics as a consequence of hydration and ECM interactions. These measurements reveal atom-specific variations in the dynamics and structure of the N-acetylglucosamine moiety of HA. We discuss possible implications for interactions that stabilize the structure of HMW-HA and facilitate its recognition by HA-binding proteins. The described methods apply similarly to the studies of the molecular structure and dynamics of HA in tumor contexts and in other biological tissues as well as HMW-HA hydrogels and nanoparticles used for biomedical and/or pharmaceutical applications.

Nuclear magnetic resonance spectroscopy to quantify major extracellular matrix components in fibro-calcific aortic valve disease

Fibro-calcific aortic valve disease (FCAVD) is a pathological condition marked by overt fibrous and calcific extracellular matrix (ECM) accumulation that leads to valvular dysfunction and left ventricular outflow obstruction. Costly valve implantation is the only approved therapy. Multiple pharmacological interventions are under clinical investigation, however, none has proven clinically beneficial. This failure of translational approaches indicates incomplete understanding of the underlying pathomechanisms and may result from a limited toolbox of scientific methods to assess the cornerstones of FCAVD: lipid deposition, fibrous and calcific ECM accumulation. In this study, we evaluated magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy to both, qualitatively and quantitatively assess these key elements of FCAVD pathogenesis. NMR spectra showed collagen, elastin, triacylglycerols, and phospholipids in human control and FCAVD tissue samples (n = 5). Calcification, measured by the hydroxyapatite content, was detectable in FCAVD tissues and in valve interstitial cells under procalcifying media conditions. Hydroxyapatite was significantly higher in FCAVD tissues than in controls (p < 0.05) as measured by 31P MAS NMR. The relative collagen content was lower in FCAVD tissues vs. controls (p < 0.05). Overall, we demonstrate the versatility of NMR spectroscopy as a diagnostic tool in preclinical FCAVD assessment.

Production of isotopically enriched high molecular weight hyaluronic acid and characterization by solid-state NMR

Hyaluronic acid (HA) is a naturally occurring polysaccharide that is abundant in the extracellular matrix (ECM) of all vertebrate cells. HA-based hydrogels have attracted great interest for biomedical applications due to their high viscoelasticity and biocompatibility. In both ECM and hydrogel applications, high molecular weight (HMW)-HA can absorb a large amount of water to yield matrices with a high level of structural integrity. To understand the molecular underpinnings of structural and functional properties of HA-containing hydrogels, few techniques are available. Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for such studies, e.g. ¹³C NMR measurements can reveal the structural and dynamical features of (HMW) HA. However, a major obstacle to ¹³C NMR is the low natural abundance of ¹³C, necessitating the generation of HMW-HA that is enriched with ¹³C isotopes. Here we present a convenient method to obtain ¹³C- and ¹⁵N-enriched HMW-HA in good yield from Streptococcus equi subsp. zooepidemicus. The labeled HMW-HA has been characterized by solution and magic angle spinning (MAS) solid-state NMR spectroscopy, as well as other methods. These results will open new ways to study the structure and dynamics of HMW-HA-based hydrogels, and interactions of HMW-HA with proteins and other ECM components, using advanced NMR techniques.

1 H-13 C cross-polarization kinetics to probe hydration-dependent organic components of bone extracellular matrix

Bone is a living tissue made up of organic proteins, inorganic minerals, and water. The organic component of bone (mainly made up of Type-I collagen) provides flexibility and tensile strength. Solid-state nuclear magnetic resonance (ssNMR) is one of the few techniques that can provide atomic-level structural insights of such biomaterials in their native state. In the present article, we employed the variable contact time cross-polarization (¹ H-¹³ C CP) kinetics experiments to study the hydration-dependent atomic-level structural changes in the bone extracellular matrix (ECM). The natural abundant ¹³ C CP intensity of the bone ECM is measured by varying CP contact time and best fitted to the nonclassical kinetic model. Different relaxation parameters were measured by the best-fit equation corresponding to the different hydration conditions of the bone ECM. The associated changes in the measured parameters due to varying levels of hydration observed at different sites of collagen protein have provided its structural arrangements and interaction with water molecules in bone ECM. Overall, the present study reveals a better understanding of the kinetics of the organic part inside the bone ECM that will help in comprehending the disease-associated pathways.

Age-related changes in cationic compositions of human cranial base bone apatite measured by X-ray energy dispersive spectroscopy (EDS) coupled with scanning electron microscope (SEM)

One of the most common scientific methods to study the chemical composition of bone matter is energy-dispersive X-ray spectroscopy (EDS). However, interpretation of the data obtained can be quite complicated and require a thorough understanding of bone structure. This is especially important when evaluating subtle changes of chemical composition, including the age-related ones. The aim of current study is to create a method of processing the obtained data that can be utilized in clinical medicine and use it to evaluate the age evolution of bone chemical composition. To achieve this goal, an elemental composition of 62 samples of cadaver compact bone, taken from the skull base (age: Me = 57.5; 21/91(min/max); Q1 = 39.5, Q3 = 73.75), was studied with EDS. We used the original method to estimate the amount of Mg²⁺ cations. We detected and confirmed an increase of Mg²⁺ cation formula amount in the bone apatite, which characterizes age-related resorption rate. Analysis of cation estimated ratio in a normative bone hydroxylapatite showed an increase of Mg²⁺ amount (R = 0.43, p = 0.0005). Also, Ca weight fraction was shown to decrease with age (R = - 0.43, p = 0.0005), which in turn confirmed the age-dependent bone decalcification. In addition, electron probe microanalysis (EPMA) and X-ray diffraction analysis (XRD) were performed. EDS data confirmed the EPMA results (R = 0.76, p = 0.001). In conclusion, the proposed method can be used in forensic medicine and provide additional data to the known trends of decalcification and change of density and crystallinity of mineral bone matter.