In situ characterization of advanced glycation end products (AGEs) in collagen and model extracellular matrix by solid state NMR

Interface-edited solid-state NMR to study cell interfaces

Cell membrane interfaces, including the glycocalyx, play a crucial role in regulating signaling and molecular interactions, yet their molecular composition remains challenging to study in intact cells. Existing techniques often require extensive sample preparation or lack specificity for probing interfacial components directly. Here, we introduce a solid-state nuclear magnetic resonance (ssNMR) tool to fingerprint the molecular structure of the cell glycocalyx in intact cells within their native environment, offering insights relevant to drug delivery, tissue engineering, and biomedical research. Building on Goldman-Shen cross-polarization (CP) experiments, which exploit proton spin diffusion to generate 13C spectra near cell membranes, our enhanced approach provides spectral information from the membrane interface and its surroundings, probing a region up to 10 nm. Using interface-edited CP (1D) and PDSD (2D) spectra, we demonstrate spectral fingerprints of the mammalian cell glycocalyx. This method opens new avenues for studying cell interfaces in a dehydrated yet native-like state, preserving membrane composition and advancing structural biology.

Adhesion of Mesenchymal Stem Cells to Glycated Collagen-Comparative Analysis of Dynamic and Static Conditions

Understanding mesenchymal stem cell (MSC) behavior on glycated collagen is crucial for advancing regenerative medicine and understanding pathological mechanisms in diseases such as diabetes, cancer, and aging. While previous research has demonstrated reduced MSC interaction with glycated collagen under static conditions due to disrupted integrin signaling, these studies did not accurately replicate the dynamic mechanical environment that MSCs encounter in vivo. Here we present a comprehensive investigation comparing adipose-derived MSC (ADMSC) behavior under both dynamic flow conditions and static adhesion, revealing unexpected temporal dynamics and challenging existing paradigms of cell-matrix interactions. Using a sophisticated microfluidic BioFlux system combined with traditional static adhesion assays, we examined ADMSC interactions with native collagen for 1-day glycated (GL1), and 5-day glycated (GL5) samples. Under flow conditions, MSCs demonstrated remarkably rapid attachment-within 3-5 min-contrasting sharply with the classical 2 h static incubation protocol. This rapid adhesion was particularly enhanced on 5-day glycated collagen, though subsequent testing revealed significantly weaker adhesion strength under shear stress compared to native collagen. Static conditions also showed a distinct pattern: increased ADMSC adhesion to glycated samples within the first 30 min, followed by a progressive decrease in adhesion and compromised cell spreading over longer periods. Atomic force microscopy (AFM) analysis revealed significant changes in collagen surface properties upon glycation. These included a substantial reduction in the negative surface charge (from ~800 to 600 mV), altered surface roughness patterns (Rrms varying from 3.0 ± 0.4 nm in native collagen to 7.70 ± 0.6 nm in GL5), and decreased elasticity (Young's modulus dropping from 34.8 ± 5.4 MPa to 2.07 ± 0.3 MPa in GL5). These physical alterations appear to facilitate rapid initial cell attachment while potentially compromising long-term stable adhesion through traditional integrin-mediated mechanisms. This study provides novel insights into the complex dynamics of MSC adhesion to glycated collagen, revealing previously unknown temporal patterns and challenging existing models of cell-matrix interactions. The findings suggest a need for revised approaches in tissue engineering and regenerative medicine, particularly in conditions where glycated collagen is prevalent.

Solid-State NMR Spectroscopy Investigation of Structural Changes of Mechanically Strained Mouse Tail Tendons

Structural tissues like tendon are subjected to repeated tensile strains in vivo and excessive strains cause irreversible changes to the tissue. Large strains affect the molecular structure and organization of the extracellular matrix, and these are the parameters that drive cell behavior, including tissue repair. Here we describe a method to perform solid-state NMR spectroscopy on in situ strained tissue samples under magic-angle spinning to achieve high-resolution NMR spectra while maintaining the tissue's native hydration state. The changes observed in the NMR spectra are interpreted using quantum mechanics molecular mechanics (QM/MM) chemical shift calculations on strained collagen triple-helix structures and consideration of changes in the distribution of molecular orientations between strained and relaxed mechanical states. We demonstrate that our tissue strain method in combination with spectral simulations can detect changes in collagen organization between tendons loaded to plastic deformation and subsequent structural relaxation in the unloaded state.

Methylglyoxal alters collagen fibril nanostiffness and surface potential

Collagen fibrils are fundamental to the mechanical strength and function of biological tissues. However, they are susceptible to changes from non-enzymatic glycation, resulting in the formation of advanced glycation end-products (AGEs) that are not reversible. AGEs accumulate with aging and disease and can adversely impact tissue mechanics and cell-ECM interactions. AGE-crosslinks have been related, on the one hand, to dysregulation of collagen fibril stiffness and damage and, on the other hand, to altered collagen net surface charge as well as impaired cell recognition sites. While prior studies using Kelvin probe force microscopy (KPFM) have shown the effect glycation has on collagen fibril surface potential (i.e., net charge), the combined effect on individual and isolated collagen fibril mechanics, hydration, and surface potential has not been documented. Here, we explore how methylglyoxal (MGO) treatment affects the mechanics and surface potential of individual and isolated collagen fibrils by utilizing atomic force microscopy (AFM) nanoindentation and KPFM. Our results reveal that MGO treatment significantly increases nanostiffness, alters surface potential, and modifies hydration characteristics at the collagen fibril level. These findings underscore the critical impact of AGEs on collagen fibril physicochemical properties, offering insights into pathophysiological mechanical and biochemical alterations with implications for cell mechanotransduction during aging and in diabetes. STATEMENT OF SIGNIFICANCE: Collagen fibrils are susceptible to glycation, the irreversible reaction of amino acids with sugars. Glycation affects the mechanical properties and surface chemistry of collagen fibrils with adverse alterations in biological tissue mechanics and cell-ECM interactions. Current research on glycation, at the level of individual collagen fibrils, is sparse and has focused either on collagen fibril mechanics, with contradicting evidence, or surface potential. Here, we utilized a multimodal approach combining Kelvin probe force (KPFM) and atomic force microscopy (AFM) to examine how methylglyoxal glycation induces structural, mechanical, and surface potential changes on the same individual and isolated collagen fibrils. This approach helps inform structure-function relationships at the level of individual collagen fibrils.

Comprehensive characterization of differential glycation in hepatocellular carcinoma using tissue proteomics with stable isotopic labeling

Glycation is a non-enzymatic posttranslational modification coming from the reaction between reducing sugars and free amino groups in proteins, where early glycation products (fructosyl-lysine, FL) and advanced glycation end products (AGEs) are formed. The occurrence of glycation and accumulation of AGEs have been closely associated with hepatocellular carcinoma (HCC). Here, we reported the characterization of differential glycation in HCC using tissue proteomics with stable isotopic labeling; early glycation-modified peptides were enriched with boronate affinity chromatography (BAC), and AGEs-modified peptides were fractionated with basic reversed-phase separation. By this integrated approach, 3717 and 1137 early and advanced glycated peptides corresponding to 4007 sites on 1484 proteins were identified with a false discovery rate (FDR) of no more than 1%. One hundred fifty-five sites were modified with both early and advanced end glycation products. Five early and 7 advanced glycated peptides were quantified to be differentially expressed in HCC tissues relative to paired adjacent tissues. Most (8 out of 10) of the proteins corresponding to the differential glycated peptides have previously been reported with dysregulation in HCC. The results together may deepen our knowledge of glycation as well as provide insights for therapeutics.

Early Stages of Ex Vivo Collagen Glycation Disrupt the Cellular Interaction and Its Remodeling by Mesenchymal Stem Cells-Morphological and Biochemical Evidence

Mesenchymal stem cells (MSCs), pivotal for tissue repair, utilize collagen to restore structural integrity in damaged tissue, preserving its organization through concomitant remodeling. The non-enzymatic glycation of collagen potentially compromises MSC communication, particularly upon advancing the process, underlying various pathologies such as late-stage diabetic complications and aging. However, an understanding of the impact of early-stage collagen glycation on MSC interaction is lacking. This study examines the fate of in vitro glycated rat tail collagen (RTC) upon exposure to glucose for 1 or 5 days in contact with MSCs. Utilizing human adipose tissue-derived MSCs (ADMSCs), we demonstrate their significantly altered interaction with glycated collagen, characterized morphologically by reduced cell spreading, diminished focal adhesions formation, and attenuated development of the actin cytoskeleton. The morphological findings were confirmed by ImageJ 1.54g morphometric analysis with the most significant drop in the cell spreading area (CSA), from 246.8 μm2 for the native collagen to 216.8 μm2 and 163.7 μm2 for glycated ones, for 1 day and 5 days, respectively, and a similar trend was observed for cell perimeter 112.9 μm vs. 95.1 μm and 86.2 μm, respectively. These data suggest impaired recognition of early glycated collagen by integrin receptors. Moreover, they coincide with the reduced fibril-like reorganization of adsorbed FITC-collagen (indicating impaired remodeling) and a presumed decreased sensitivity to proteases. Indeed, confirmatory assays reveal diminished FITC-collagen degradation for glycated samples at 1 day and 5 days by attached cells (22.8 and 30.4%) and reduced proteolysis upon exogenous collagenase addition (24.5 and 40.4%) in a cell-free system, respectively. The mechanisms behind these effects remain uncertain, although differential scanning calorimetry confirms subtle structural/thermodynamic changes in glycated collagen.

AGEs accumulation with vascular complications, glycemic control and metabolic syndrome: A narrative review

BACKGROUND

Multiple pathogenetic mechanisms are involved in the genesis of various microvascular and macrovascular complications of diabetes mellitus. Of all these, advanced glycation end products (AGEs) have been strongly implicated.

OBJECTIVES

The present narrative review aims to summarize the available literature on the genesis of AGEs and their potential role in the causation of both micro- and macrovascular complications of diabetes mellitus.

RESULTS

Uncontrolled hyperglycemia triggers the formation of AGEs through non-enzymatic glycation reactions between reducing sugars and proteins, lipids, or nucleic acids. AGEs accumulate in bloodstream and bodily tissues under chronic hyperglycemia. AGEs create irreversible cross-linkages of various intra- and extracellular molecules and activate the receptor for advanced glycation end products (RAGE), which stimulates downstream signaling pathways that generate reactive oxygen species (ROS) and contribute to oxidative stress. Additionally, intracellular glycation of mitochondrial respiratory chain proteins by AGEs contributes to the further generation of ROS, which, in turn, sets a vicious cycle that further promotes the production of endogenous AGEs. Through these pathways, AGEs play a principal role in the pathogenesis of various diabetic complications, including diabetic retinopathy, nephropathy, neuropathy, bone disease, atherosclerosis and non-alcoholic fatty liver disease. Multiple clinical studies and meta-analyses have revealed a positive association between tissue or circulating levels of AGEs and development of various diabetic complications. Besides, exogenous AGEs, primarily those derived from diets, promote insulin resistance, obesity, and metabolic syndrome.

CONCLUSIONS

AGEs, triggered by chronic hyperglycemia, play a pivotal role in the pathogenesis of various complications of diabetes mellitus.

Focusing on the Native Matrix Proteins in Calcific Aortic Valve Stenosis

Calcific aortic valve stenosis (CAVS) is a widespread valvular heart disease affecting people in aging societies, primarily characterized by fibrosis, inflammation, and progressive calcification, leading to valve orifice stenosis. Understanding the factors associated with CAVS onset and progression is crucial to develop effective future pharmaceutical therapies. In CAVS, native extracellular matrix proteins modifications, play a significant role in calcification in vitro and in vivo. This work aimed to review the evidence on the alterations of structural native extracellular matrix proteins involved in calcification development during CAVS and highlight its link to deregulated biomechanical function.

Solid‐state NMR ( SSNMR ) Characterization of Osteoblast from Mesenchymal Stromal Cell Differentiation to Osteoblast Mineralization

Solid‐state nuclear magnetic resonance (SSNMR), a technique capable of studying solid or semisolid biological samples, was first applied to study the cell differentiation and mineralization using the whole‐cell sample. Mesenchymal stromal cells (MSCs) with multipotent differentiation capacity were induced to differentiate into osteoblasts. The whole differentiation process, osteoblast mineralization and the mineral maturation, was investigated using SSNMR, providing intact, atomic level information on the cellular mineral structural transformation. Our research indicated the extent of osteoblast mineralization could vary significantly for different cell populations whereas the difference was not easily shown by other means of characterization. The SSNMR spectra revealed hydroxylapatite (or hydroxyapatite [HAP]) formation around 2 to 4 weeks after osteogenic induction for MSCs with a high differentiation potency. The early mineral phase deposit before HAP formation contained a high amount of HPO₄²⁻. The structures of minerals in the extracellular matrix (ECM) of osteoblasts could evolve for a period of time, even after the incubation of cells has been stopped. This observation was only possible by studying the sample in an intact state, where ECM was not disturbed. These findings improved our understanding of MSCs, which had wide applications in bone regeneration and tissue engineering. Meanwhile, this work demonstrated the advantage of studying these cellular systems as a whole without any mineral extraction, which had been largely overlooked. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

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

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Solid-state NMR spectroscopy probes molecular conformation and dynamics in intact ECM.

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Collagen conformational dynamics has roles in mechanical properties of fibrils and cell adhesion.

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Solid-state NMR spectroscopy has shed new light on  the chemical structure of bone mineral.

Solid-state NMR spectroscopy has played an important role in multidisciplinary studies of the extracellular matrix. Here we review how solid-state NMR has been used to probe collagen molecular conformations, dynamics, post-translational modifications and non-enzymatic chemical changes, and in calcified tissues, the molecular structure of bone mineral and its interface with collagen. We conclude that NMR spectroscopy can deliver vital information that in combination with data from structural imaging techniques, can result in significant new insight into how the extracellular matrix plays its multiple roles.

Glycation changes molecular organization and charge distribution in type I collagen fibrils

Collagen fibrils are central to the molecular organization of the extracellular matrix (ECM) and to defining the cellular microenvironment. Glycation of collagen fibrils is known to impact on cell adhesion and migration in the context of cancer and in model studies, glycation of collagen molecules has been shown to affect the binding of other ECM components to collagen. Here we use TEM to show that ribose-5-phosphate (R5P) glycation of collagen fibrils - potentially important in the microenvironment of actively dividing cells, such as cancer cells - disrupts the longitudinal ordering of the molecules in collagen fibrils and, using KFM and FLiM, that R5P-glycated collagen fibrils have a more negative surface charge than unglycated fibrils. Altered molecular arrangement can be expected to impact on the accessibility of cell adhesion sites and altered fibril surface charge on the integrity of the extracellular matrix structure surrounding glycated collagen fibrils. Both effects are highly relevant for cell adhesion and migration within the tumour microenvironment.

Longitudinal Relaxation Optimization Enhances 1 H-Detected HSQC in Solid-State NMR Spectroscopy on Challenging Biological Systems

Solid-state (SS) NMR spectroscopy is a powerful technique for studying challenging biological systems, but it often suffers from low sensitivity. A longitudinal relaxation optimization scheme to enhance the signal sensitivity of HSQC experiments in SSNMR spectroscopy is reported. Under the proposed scheme, the ¹ H spins of ¹ H-X (¹⁵ N or ¹³ C) are selected for signal acquisition, whereas other vast ¹ H spins are flipped back to the axis of the static magnetic field to accelerate the spin recovery of the observed ¹ H spins, resulting in enhanced sensitivity. Three biological systems are used to evaluate this strategy, including a seven-transmembrane protein, an RNA, and a whole-cell sample. For all three samples, the proposed scheme largely shortens the effective ¹ H longitudinal relaxation time and results in a 1.3-2.5-fold gain in sensitivity. The selected systems are representative of challenging biological systems for observation by means of SSNMR spectroscopy; thus indicating the general applicability of this method, which is particularly important for biological samples with a short lifetime or with limited sample quantities.

Detection of nucleic acids and other low abundance components in native bone and osteosarcoma extracellular matrix by isotope enrichment and DNP-enhanced NMR

Sensitivity enhancement by isotope enrichment and DNP NMR enables detection of minor but biologically relevant species in native intact bone, including nucleic acids, choline from phospholipid headgroups, and histidinyl and hydroxylysyl groups. Labelled matrix from the aggressive osteosarcoma K7M2 cell line confirms the assignments of nucleic acid signals arising from purine, pyrimidine, ribose, and deoxyribose species. Detection of these species is an important and necessary step in elucidating the atomic level structural basis of their functions in intact tissue.

Towards elucidating their biological roles in intact tissue, DNP NMR reveals nucleic acids, and other important low abundance biomolecules in a complex biomaterial, bone, and in cancer extracellular matrix.

Essential but sparse collagen hydroxylysyl post-translational modifications detected by DNP NMR

The sparse but functionally essential post-translational collagen modification 5-hydroxylysine can undergo further transformations, including crosslinking, O-glycosylation, and glycation. Dynamic nuclear polarization (DNP) and stable isotope enriched lysine incorporation provide sufficient solid-state NMR sensitivity to identify these adducts directly in skin and vascular smooth muscle cell extracellular matrix (ECM), without extraction procedures, by comparison with chemical shifts of model compounds. Thus, DNP provides access to the elucidation of structural consequences of collagen modifications in intact tissue.

Collagen Structure-Function Relationships from Solid-State NMR Spectroscopy

The extracellular matrix of a tissue is as important to life as the cells within it. Its detailed molecular structure defines the environment of a tissue's cells and thus their properties, including differentiation and metabolic status. Collagen proteins are the major component of extracellular matrices. Self-assembled collagen fibrils provide both specific mechanical properties to handle external stresses on tissues and, at the molecular level, well-defined protein binding sites to interact with cells. How the cell-matrix interactions are maintained against the stresses on the tissue is an important and as yet unanswered question. Similarly, how collagen molecular and fibrillar structures change in aging and disease is a crucial open question. Solid-state NMR spectroscopy offers insight into collagen molecular conformation in intact in vivo and in vitro tissues, and in this Account we review how NMR spectroscopy is beginning to provide answers to these questions. In vivo ¹³C,¹⁵N labeling of the extracellular matrix has given insight into collagen molecular dynamics and generated multidimensional NMR "fingerprints" of collagen molecular structure that allow comparison of local collagen conformation between tissues. NMR studies have shown that charged collagen residues (Lys, Arg) adopt extended-side-chain conformations in the fibrillar structure to facilitate charge-charge interactions between neighboring collagen molecules, while hydrophobic residues (Leu, Ile) fold along the collagen molecular axis to minimize the hydrophobic area exposed to surrounding water. Detailed NMR and molecular modeling work has shown that the abundant Gly-Pro-Hyp (Hyp = hydroxyproline) triplets in collagen triple helices confer well-defined flexibility because the proline is conformationally metastable, in contrast to the expectation that these triplets confer structural rigidity. The alignment of the Gly-Pro-Hyp triplets within the fibril structure means that the Gly-Pro-Hyp molecular flexibility generates fibril flexibility. The fibrillar bands of Gly-Pro-Hyp are highly correlated with collagen ligand binding sites, leading to the hypothesis that the fibril alignment of Gly-Pro-Hyp triplets is essential to protect collagen-ligand binding against external stresses on the tissue. Non-enzymatic chemistry between collagen side-chain amine groups (Lys, Arg) and reducing sugars-glycation-is an important source of matrix structural change in aging and disease. Glycation leads to stiffening of collagen fibrils, which is widely speculated to be the result of intermolecular cross-linking. The chemistry of non-enzymatic glycation has been extensively detailed through NMR studies and has been shown to lead to side-chain modifications as the majority reaction products, rather than intermolecular cross-links, with resultant molecular misalignment in the fibrils. Thus, a picture is beginning to emerge in which collagen glycation causes stiffening through misalignment of collagen molecular flexible regions rather than intermolecular cross-linking, meaning that new thinking is needed on how to alleviate collagen structural changes in aging and disease.