Deposition of apatite in mineralizing vertebrate extracellular matrices: A model of possible nucleation sites on type I collagen

Zonal Characteristics of Collagen Ultrastructure and Responses to Mechanical Loading in Articular Cartilage

The biomechanical properties of articular cartilage arise from a complex bioenvironment comprising hierarchically organised collagen networks within the extracellular matrix (ECM) that interact with the proteoglycan-rich interstitial fluid. This network features a depth-dependent fibril organisation across different zones. Understanding how collagen fibrils respond to external loading is key to elucidating the mechanisms behind lesion formation and managing degenerative conditions like osteoarthritis. This study employs polarisation-resolved second harmonic generation (pSHG) microscopy to quantify the ultrastructural organisation of collagen fibrils and their spatial gradient along the depth of bone-cartilage explants under a close-to-in vivo condition. By combining with in-situ loading, we examined the responses of collagen fibrils by quantifying changes in their principal orientation and degree of alignment. The spatial gradient and heterogeneity of collagen organisation were captured at high resolution (1 μm) along the longitudinal plane of explants (0.5 mm by 2 mm). Zone-specific ultrastructural characteristics were quantified to aid in defining zonal borders, revealing consistent zonal proportions with varying overall thicknesses. Under compression, the transitional zone exhibited the most significant re-organisation of collagen fibrils. It initially allowed large deformation through the re-orientation of fibrils, which then tightened fibril alignment to prevent excessive deformation, indicating a dynamic adaptation mechanism in response to increasing strain levels. Our results provide comprehensive, zone-specific baselines of cartilage ultrastructure and micromechanics, crucial for investigating the onset and progression of degenerative conditions, setting therapeutic intervention targets, and guiding cartilage repair and regeneration efforts. STATEMENT OF SIGNIFICANCE: Achieved unprecedented quantification of the spatial gradient and heterogeneity of collagen ultrastructural organisation at a high resolution (1 μm) along the full depth of the longitudinal plane of osteochondral explants (0.5 mm by 2 mm) under close-to-in vivo condition. Suggested new anatomical landmarks based on ultrastructural features for determining zonal borders and found consistent zonal proportions in explants with different overall thicknesses. Demonstrated that collagen fibrils initially respond by reorienting themselves at low strain levels, playing a significant role in cartilage deformation, particularly within the transitional zone. At higher strain levels, more collagen fibrils re-aligned, indicating a dynamic shift in the response mechanism at varying strain levels.

A collagen-based laboratory model to mimic sex-specific features of calcific aortic valve disease

Calcific aortic valve disease (CAVD) shows in the deposition of calcium phosphates in the collagen-rich layer of the valve leaflets. This stiffens the leaflets and eventually leads to heart failure. Recent research suggests that CAVD follows sex-specific pathways: at the same severity of the disease, women tend to have fewer and less crystalline calcifications, and the phases of their calcifications are decidedly different than those of men; namely, dicalcium phosphate dihydrate (DCPD) - one of the mineral phases in CAVD - occurs almost exclusively in females. Furthermore, the morphologies of heart valve calcifications might be sex-specific, but the sex dependence of the morphologies has not been systematically investigated. Herein, we first show that male CAVD patients have more compact and less fibrous calcifications than females, establishing sex-dependent morphological features of heart valve calcification. We then build a model that recapitulates the sex differences of the calcifications in CAVD, which is based on a collagen gel that we calcify in simulated body fluid with varying fetuin A concentrations. With increasing fetuin A concentration, the calcifications become less crystalline and more fibrous, and more DCPD deposits in the collagen matrix, resembling the physicochemical characteristics of the calcifications in female valves. Lower fetuin A concentrations give rise to a model that replicates male-specific mineral characteristics. The models could be used to develop sex-specific detection and treatment methods for CAVD. STATEMENT OF SIGNIFICANCE: Although calcific aortic valve disease (CAVD) affects ∼10 million people globally, researchers have only discovered recently that the disease follows sex-specific pathways, and many of its sex-specific features remain unknown. To further our understanding of sex differences in CAVD and to develop better detection and treatment methods, there is an urgent need to establish models for CAVD that account for its sex-specific manifestations. In this study, we first show that CAVD calcifications in men and women take on different morphologies. Second, we present a model that can replicate physicochemical calcification characteristics of male or female valves, including morphology, and that can help to develop sex-specific detection and treatment methods for CAVD.

The molecular basis of octocoral calcification revealed by genome and skeletal proteome analyses

The ability of octocorals and stony corals to deposit calcium carbonate (CaCO3) has contributed to their ecological success. Whereas stony corals possess a homogeneous aragonite skeleton, octocorals have developed distinct skeletal structures composed of different CaCO3 polymorphs and a skeletal organic matrix. Nevertheless, the molecular basis of skeletal structure formation in octocorals remains inadequately understood. Here, we sequenced the genomes and skeletal proteomes of two calcite-forming octocorals, namely Paragorgia papillata and Chrysogorgia sp. The assembled genomes sizes were 618.13 Mb and 781.04 Mb for P. papillata and Chrysogorgia sp., respectively, with contig N50s of 2.67 Mb and 2.61 Mb. Comparative genomic analyses identified 162 and 285 significantly expanded gene families in the genomes of P. papillata and Chrysogorgia sp., respectively, which are primarily associated with biomineralization and immune response. Furthermore, comparative analyses of skeletal proteomes demonstrated that corals with different CaCO3 polymorphs share a fundamental toolkit comprising cadherin, von Willebrand factor type A, and carbonic anhydrase domains for calcified skeleton deposition. In contrast, collagen is abundant in the calcite-forming octocoral skeletons but occurs rarely in aragonitic stony corals. Additionally, certain collagens have developed domains related to matrix adhesion and immunity, which may confer novel genetic functions in octocoral calcification. These findings enhance our understanding of the diverse forms of coral biomineralization processes and offer preliminary insights into the formation and evolution of the octocoral skeleton.

Aspartic Acid Binding on Hydroxyapatite Nanoparticles with Varying Morphologies Investigated by Solid-State NMR Spectroscopy and Molecular Dynamics Simulation

Hydroxyapatite (HAP) exhibits a highly oriented hierarchical structure in biological hard tissues. The formation and selective crystalline orientation of HAP is a process that involves functional biomineralization proteins abundant in acidic residues. To obtain insights into the process of HAP mineralization and acidic residue binding, synthesized HAP with specific lattice planes including (001), (100), and (011) are structurally characterized following the adsorption of aspartic acid (Asp). The adsorption affinity of Asp on HAP surfaces is evaluated quantitatively and demonstrates a high dependency on the HAP morphological form. Among the synthesized HAP nanoparticles (NPs), Asp exhibits the strongest adsorption affinity to short HAP nanorods, which are composed of (100) and (011) lattice planes, followed by nanosheets with a preferential expression of the (001) facet, to which Asp displays a similar but slightly lower binding affinity. HAP nanowires, with the (100) lattice plane preferentially developed, show significantly lower affinity to Asp and evidence of multilayer adsorption compared to the previous two types of HAP NPs. A combination of solid-state NMR (SSNMR) techniques including 13C and 15N CP-MAS, relaxation measurements and 13C-31P Rotational Echo DOuble Resonance (REDOR) is utilized to characterize the molecular structure and dynamics of Asp-HAP bionano interfaces with 13C- and 15N-enriched Asp. REDOR is used to determine 13C-31P internuclear distances, providing insight into the Asp binding geometry where stronger 13C-31P dipolar couplings correlate with binding affinity determined from Langmuir isotherms. The carboxyl sites are identified as the primary binding groups, facilitated by their interaction with surface calcium sites. The Asp chelation conformations revealed by SSNMR are further refined with molecular dynamics (MD) simulation where specific models strongly agree between the SSNMR and MD models for the various surfaces.

Biology of bone mineralization and ectopic calcifications: the same actors for different plays

Bone has several crucial functions. It is essential for locomotion and allows our body to stand erect against gravity. A mismatch between the mechanical stresses applied to it and its mechanical resistance leads to fractures. Bone also has numerous endocrine functions. It acts as a reservoir for minerals such as calcium and phosphorus, making it the target of calciotropic hormones that mobilize these minerals, particularly calcium, according to the body's needs. Additionally, bone secretes hormones, notably fibroblast growth factor 23 (FGF23), which regulates urinary excretion of phosphate and the bioavailability of active vitamin D. Bone mineralization is the process that facilitates the organized deposition of minerals in the bone matrix, providing rigidity and appropriate mechanical resistance. This process is compromised in genetically related bone mineralization disorders, such as those causing hypophosphatemia or hypophosphatasia. Conversely, calcification can be pathological, affecting soft tissues like the blood vessels, as seen in generalized arterial calcification of infancy (GACI) or arterial calcification due to CD73 deficiency (ACDC). The aim of this article is to first present the composition and structure of the mineralized bone matrix, to review the current understanding of the molecular mechanisms of mineralization, and finally to discuss the conditions associated with ectopic calcification and the underlying mechanisms.

Small Charged Molecule-Mediated Fibrillar Mineralization: Implications for Ectopic Calcification

Numerous small biomolecules exist in the human body and play roles in various biological and pathological processes. Small molecules are believed not to induce intrafibrillar mineralization alone. They are required to work in synergy with noncollagenous proteins (NCPs) and their analogs, e.g. polyelectrolytes, for inducing intrafibrillar mineralization, as the polymer-induced liquid-like precursor (PILP) process has been well-documented. In this study, we demonstrate that small charged molecules alone, such as sodium tripolyphosphate, sodium citrate, and (3-aminopropyl) triethoxysilane, could directly mediate fibrillar mineralization. We propose that small charged molecules might be immobilized in collagen fibrils to form the polyelectrolyte-like collagen complex (PLCC) via hydrogen bonds. The PLCC could attract CaP precursors along with calcium and phosphate ions for inducing mineralization without any polyelectrolyte additives. The small charged molecule-mediated mineralization process was evidenced by Cryo-TEM, AFM, SEM, FTIR, ICP-OES, etc., as the PLCC exhibited both characteristic features of collagen fibrils and polyelectrolyte with increased charges, hydrophilicity, and density. This might hint at one mechanism of pathological biomineralization, especially for understanding the ectopic calcification process.

Dentine surface modification and remineralization induced by bioactive toothpastes

OBJECTIVE

In this study, dentine surface was analysed through Environmental-scanning-electron-microscopy (ESEM) with energy-dispersive-X-ray-spectrometry (EDX) and Fourier-transform-infrared-spectroscopy (FTIR) with attenuated total-reflectance (ATR) to assess the morpho-chemical changes and variations in mineralization degree after demineralizing treatment, after five toothpastes application (HA & Citrate toothpaste, Zinc-HA toothpaste, Calcium Sodium Phosphosilicate toothpaste, Arginine & Calcium carbonate toothpaste, Colgate-Triple-Action, and Control toothpaste), after soaking in artificial saliva and after citric acid attack.

METHODS

Ca/P, Ca/N and P/N ratios were calculated from EDX atomic data to evaluate the mineralization degree of dentine surface. The IR calcium phosphate (CaP)/collagen and carbonate/collagen ratios has been evaluated to assess the remineralization changes in dentine; the carbonate/collagen IR ratio was calculated to identify the nucleation of B-type-carbonated apatite and calcium carbonate.

RESULTS

ESEM-EDX and ATR-FTIR showed residuals of toothpastes after the treatments in all cases, with a general increase in the mineralization degree after soaking in artificial saliva and a decrease after acid attack. Treatment with Arginine & Calcium carbonate toothpaste showed the highest Ca/P value after treatment (Ca/P 1.62) and acid attack (Ca/P 1.5) in confirmation, IR showed the highest amount of carbonate after treatment and soaking in artificial saliva. Arginine and calcium carbonate toothpaste and HA and citrate toothpaste remained to a higher extent on the dentine surface and revealed a higher remineralization activity. These formulations showed higher resistance to demineralization attack, as demonstrated by a higher ICaP/IAmide II intensity ratio than those obtained after EDTA treatment.

CONCLUSIONS

Toothpastes that remained to a higher extent on dentine surface (arginine and calcium carbonate toothpaste in particular) were more able to promote remineralization. The formed calcium phosphate (CaPs) phase was intimately bound to dentine rather than a simple deposit.

Advancing dentin remineralization: Exploring amorphous calcium phosphate and its stabilizers in biomimetic approaches

OBJECTIVE

This review elucidates the mechanisms underpinning intrafibrillar mineralization, examines various amorphous calcium phosphate (ACP) stabilizers employed in dentin's intrafibrillar mineralization, and addresses the challenges encountered in clinical applications of ACP-based bioactive materials.

METHODS

The literature search for this review was conducted using three electronic databases: PubMed, Web of Science, and Google Scholar, with specific keywords. Articles were selected based on inclusion and exclusion criteria, allowing for a detailed examination and summary of current research on dentin remineralization facilitated by ACP under the influence of various types of stabilizers.

RESULTS

This review underscores the latest advancements in the role of ACP in promoting dentin remineralization, particularly intrafibrillar mineralization, under the regulation of various stabilizers. These stabilizers predominantly comprise non-collagenous proteins, their analogs, and polymers. Despite the diversity of stabilizers, the mechanisms they employ to enhance intrafibrillar remineralization are found to be interrelated, indicating multiple driving forces behind this process. However, challenges remain in effectively designing clinically viable products using stabilized ACP and maximizing intrafibrillar mineralization with limited materials in practical applications.

SIGNIFICANCE

The role of ACP in remineralization has gained significant attention in dental research, with substantial progress made in the study of dentin biomimetic mineralization. Given ACP's instability without additives, the presence of ACP stabilizers is crucial for achieving in vitro intrafibrillar mineralization. However, there is a lack of comprehensive and exhaustive reviews on ACP bioactive materials under the regulation of stabilizers. A detailed summary of these stabilizers is also instrumental in better understanding the complex process of intrafibrillar mineralization. Compared to traditional remineralization methods, bioactive materials capable of regulating ACP stability and controlling release demonstrate immense potential in enhancing clinical treatment standards.

Effects of mineralization on the hierarchical organization of collagen-a synchrotron X-ray scattering and polarized second harmonic generation study

The process of mineralization fundamentally alters collagenous tissue biomechanics. While the structure and organization of mineral particles have been widely studied, the impact of mineralization on collagen matrix structure, particularly at the molecular scale, requires further investigation. In this study, synchrotron X-ray scattering (XRD) and polarization-resolved second harmonic generation microscopy (pSHG) were used to study normally mineralizing turkey leg tendon in tissue zones representing different stages of mineralization. XRD data demonstrated statistically significant differences in collagen D-period, intermolecular spacing, fibril and molecular dispersion and relative supramolecular twists between non-mineralizing, early mineralizing and late mineralizing zones. pSHG analysis of the same tendon zones showed the degree of collagen fibril organization was significantly greater in early and late mineralizing zones compared to non-mineralizing zones. The combination of XRD and pSHG data provide new insights into hierarchical collagen-mineral interactions, notably concerning possible cleavage of intra- or interfibrillar bonds, occlusion and reorganization of collagen by mineral with time. The complementary application of XRD and fast, label-free and non-destructive pSHG optical measurements presents a pathway for future investigations into the dynamics of molecular scale changes in collagen in the presence of increasing mineral deposition.

Elucidating the role of diverse mineralisation paradigms on bone biomechanics - a coarse-grained molecular dynamics investigation

Bone as a hierarchical composite structure plays a myriad of roles in vertebrate skeletons including providing the structural stability of the body. Despite this critical role, the mechanical behaviour at the sub-micron levels of bone's hierarchy remains poorly understood. At this scale, bone is composed of Mineralised Collagen Fibrils (MCF) embedded within an extra-fibrillar matrix that consists of hydroxyapatite minerals and non-collagenous proteins. Recent experimental studies hint at the significance of the extra-fibrillar matrix in providing the bone with the stiffness and ductility needed to serve its structural roles. However, due to limited resolution of experimental tools, it is not clear how the arrangement of minerals, and in particular their relative distribution between the intra- and extra-fibrillar space contribute to bone's remarkable mechanical properties. In this study, a Coarse Grained Molecular Dynamics (CGMD) framework was developed to study the mechanical properties of MCFs embedded within an extra-fibrillar mineral matrix and the precise roles extra- and intra-fibrillar mineralisation on the load-deformation response was investigated. It was found that the presence of extra-fibrillar mineral resulted in the development of substantial residual stress in the system, by limiting MCF shortening that took place during intra-fibrillar mineralisation, resulting in substantial compressive residual stresses in the extra-fibrillar mineral phase. The simulation results also revealed the crucial role of extra-fibrillar mineralisation in determining the elastic response of the Extrafibrillar mineralised MCF (EFM-MCF) system up to the yield point, while the fibrillar collagen affected the post-yield behaviour. When physiological levels of mineralisation were considered, the mechanical response of the EFM-MCF systems was characterised by high ductility and toughness, with micro-cracks being distributed across the extra-fibrillar matrix, and MCFs effectively bridging these cracks leading to an excellent combination of strength and toughness. Together, these results provide novel insight into the deformation mechanisms of an EFM-MCF system and highlight that this universal building block, which forms the basis for lamellar bone, can provide an excellent balance of stiffness, strength and toughness, achieving mechanical properties that are far beyond the capabilities of the individual constituents acting alone.

ACP-Mediated Phase Transformation for Collagen Mineralization: A New Understanding of the Mechanism

Despite significant efforts utilizing advanced technologies, the contentious debate surrounding the intricate mechanism underlying collagen fibril mineralization, particularly with regard to amorphous precursor infiltration and phase transformation, persists. This work proposes an amorphous calcium phosphate (ACP)-mediated pathway for collagen fibril mineralization and utilizing stochastic optical reconstruction microscopy technology, and has experimentally confirmed for the first time that the ACP nanoparticles can infiltrate inside collagen fibrils. Subsequently, the ACP-mediated phase transformation occurs within collagen fibrils to form HAP crystallites, and significantly enhances the mechanical properties of the mineralized collagen fibrils compared to those achieved by the calcium phosphate ion (CPI)-mediated mineralization and resembles the natural counterpart. Furthermore, demineralized dentin can be effectively remineralized through ACP-mediated mineralization, leading to complete restoration of its mechanical properties. This work provides a new paradigm of collagen mineralization via particle-mediated phase transformation, deepens the understanding of the mechanism behind the mineralization of collagen fibrils, and offers a new strategy for hard tissue repair.

Personalized 3D-Printed Scaffolds with Multiple Bioactivities for Bioroot Regeneration

Recent advances in 3D printing offer a prospective avenue for producing transplantable human tissues with complex geometries; however, the appropriate 3D-printed scaffolds possessing the biological compatibility for tooth regeneration remain unidentified. This study proposes a personalized scaffold of multiple bioactivities, including induction of stem cell proliferation and differentiation, biomimetic mineralization, and angiogenesis. A brand-new bioink system comprising a biocompatible and biodegradable polymer is developed and reinforced with extracellular matrix generated from dentin tissue (treated dentin matrix, TDM). Adding TDM optimizes physical properties including microstructure, hydrophilicity, and mechanical strength of the scaffolds. Proteomics analysis reveals that the released proteins of the 3D-printed TDM scaffolds relate to multiple biological processes and interact closely with each other. Additionally, 3D-printed TDM scaffolds establish a favorable microenvironment for cell attachment, proliferation, and differentiation in vitro. The 3D-printed TDM scaffolds are proangiogenic and facilitate whole-thickness vascularization of the graft in a subcutaneous model. Notably, the personalized TDM scaffold combined with dental follicle cells mimics the anatomy and physiology of the native tooth root three months after in situ transplantation in beagles. The remarkable in vitro and in vivo outcomes suggest that the 3D-printed TDM scaffolds have multiple bioactivities and immense clinical potential for tooth-loss therapy.

Synergistic effect of ion-releasing fillers on the remineralization and mechanical properties of resin-dentin bonding interfaces

In modern restorative dentistry, adhesive resin materials are vital for achieving minimally invasive, esthetic, and tooth-preserving restorations. However, exposed collagen fibers are found in the hybrid layer of the resin-dentin bonding interface due to incomplete resin penetration. As a result, the hybrid layer is susceptible to attack by internal and external factors such as hydrolysis and enzymatic degradation, and the durability of dentin bonding remains limited. Therefore, efforts have been made to improve the stability of the resin-dentin interface and achieve long-term clinical success. New ion-releasing adhesive resin materials are synthesized by introducing remineralizing ions such as calcium and phosphorus, which continuously release mineral ions into the bonding interface in resin-bonded restorations to achieve dentin biomimetic remineralization and improve bond durability. As an adhesive resin material capable of biomimetic mineralization, maintaining excellent bond strength and restoring the mechanical properties of demineralized dentin is the key to its function. This paper reviews whether ion-releasing dental adhesive materials can maintain the mechanical properties of the resin-dentin bonding interface by supplementing the various active ingredients required for dentin remineralization from three aspects: phosphate, silicate, and bioactive glass.

Role of carboxylic organic molecules in interfibrillar collagen mineralization

Bone is a composite material made up of inorganic and organic counterparts. Most of the inorganic counterpart accounts for calcium phosphate (CaP) whereas the major organic part is composed of collagen. The interfibrillar mineralization of collagen is an important step in the biomineralization of bone and tooth. Studies have shown that synthetic CaP undergoes auto-transformation to apatite nanocrystals before entering the gap zone of collagen. Also, the synthetic amorphous calcium phosphate/collagen combination alone is not capable of initiating apatite nucleation rapidly. Therefore, it was understood that there is the presence of a nucleation catalyst obstructing the auto-transformation of CaP before entering the collagen gap zone and initiating rapid nucleation after entering the collagen gap zone. Therefore, studies were focused on finding the nucleation catalyst responsible for the regulation of interfibrillar collagen mineralization. Organic macromolecules and low-molecular-weight carboxylic compounds are predominantly present in the bone and tooth. These organic compounds can interact with both apatite and collagen. Adsorption of the organic compounds on the apatite nanocrystal governs the nucleation, crystal growth, lattice orientation, particle size, and distribution. Additionally, they prevent the auto-transformation of CaP into apatite before entering the interfibrillar compartment of the collagen fibril. Therefore, many carboxylic organic compounds have been utilized in developing CaP. In this review, we have covered different carboxylate organic compounds governing collagen interfibrillar mineralization.

Advances in biomimetic collagen mineralisation and future approaches to bone tissue engineering

With an ageing world population and ~20% of adults in Europe being affected by bone diseases, there is an urgent need to develop advanced regenerative approaches and biomaterials capable to facilitate tissue regeneration while providing an adequate microenvironment for cells to thrive. As the main components of bone are collagen and apatite mineral, scientists in the tissue engineering field have attempted in combining these materials by using different biomimetic approaches to favour bone repair. Still, an ideal bone analogue capable of mimicking the distinct properties (i.e., mechanical properties, degradation rate, porosity, etc.) of cancellous bone is to be developed. This review seeks to sum up the current understanding of bone tissue mineralisation and structure while providing a critical outlook on the existing biomimetic strategies of mineralising collagen for bone tissue engineering applications, highlighting where gaps in knowledge exist.

Collagen piezoelectricity in osteogenesis imperfecta and its role in intrafibrillar mineralization

Intrafibrillar mineralization plays a critical role in attaining desired mechanical properties of bone. It is well known that amorphous calcium phosphate (ACP) infiltrates into the collagen through the gap regions, but its underlying driving force is not understood. Based on the authors’ previous observations that a collagen fibril has higher piezoelectricity at gap regions, it was hypothesized that the piezoelectric heterogeneity of collagen helps ACP infiltration through the gap. To further examine this hypothesis, the collagen piezoelectricity of osteogenesis imperfecta (OI), known as brittle bone disease, is characterized by employing Piezoresponse Force Microscopy (PFM). The OI collagen reveals similar piezoelectricity between gap and overlap regions, implying that losing piezoelectric heterogeneity in OI collagen results in abnormal intrafibrillar mineralization and, accordingly, losing the benefit of mechanical heterogeneity from the fibrillar level. This finding suggests a perspective to explain the ACP infiltration, highlighting the physiological role of collagen piezoelectricity in intrafibrillar mineralization.

Lysyl hydroxylase 2 mediated collagen post-translational modifications and functional outcomes

Lysyl hydroxylase 2 (LH2) is a member of LH family that catalyzes the hydroxylation of lysine (Lys) residues on collagen, and this particular isozyme has been implicated in various diseases. While its function as a telopeptidyl LH is generally accepted, several fundamental questions remain unanswered: 1. Does LH2 catalyze the hydroxylation of all telopeptidyl Lys residues of collagen? 2. Is LH2 involved in the helical Lys hydroxylation? 3. What are the functional consequences when LH2 is completely absent? To answer these questions, we generated LH2-null MC3T3 cells (LH2KO), and extensively characterized the type I collagen phenotypes in comparison with controls. Cross-link analysis demonstrated that the hydroxylysine-aldehyde (Hyl^ald)-derived cross-links were completely absent from LH2KO collagen with concomitant increases in the Lys^ald-derived cross-links. Mass spectrometric analysis revealed that, in LH2KO type I collagen, telopeptidyl Lys hydroxylation was completely abolished at all sites while helical Lys hydroxylation was slightly diminished in a site-specific manner. Moreover, di-glycosylated Hyl was diminished at the expense of mono-glycosylated Hyl. LH2KO collagen was highly soluble and digestible, fibril diameters were diminished, and mineralization impaired when compared to controls. Together, these data underscore the critical role of LH2-catalyzed collagen modifications in collagen stability, organization and mineralization in MC3T3 cells.

Promotion of collagen mineralization and dentin repair by succinates

Biomineralization of collagen fibers is regulated by non-collagenous proteins and small biomolecules, which are essential in bone and teeth formation. In particular, small biomolecules such as succinic acid (SA) exist at a high level in hard tissues, but their role is yet unclear. Here, our work demonstrated that SA could significantly promote intrafibrillar mineralization in two- and three-dimensional collagen models, where the relative mineralization rate was 16 times faster than the control group. Furthermore, the FTIR spectra and isothermal experimental results showed that collagen molecules could interact with SA via a hydrogen bond and that the interaction energy was about 4.35 kJ mol^-1. As expected, the SA-pretreated demineralized dentin obtained full remineralization within two days, whereas it took more than four days in the control group, and their mechanical properties were considerably enhanced compared with those of the demineralized one. The possible mechanism of the promotion effect of SA was ultimately illustrated, with SA modification strengthening the capacity of the collagen matrix to attract more calcium ions, which might create a higher local concentration that could accelerate the mineralization of collagen fibers. These findings not only advance the understanding of the vital role of small biomolecules in collagen biomineralization but also facilitate the development of an effective strategy to repair hard tissues.

Remineralization of human dentin type I collagen fibrils induced by carboxylated polyamidoamine dendrimer/amorphous calcium phosphate nanocomposite: an in vitro study

Intrafibrillar mineralization of type I collagen fibrils is of great significance in dental remineralization, which is the key of caries prevention and treatment. Herein, two substances that have the remineralization ability, carboxylated polyamidoamine dendrimer (PAMAM-COOH) and nano-sized amorphous calcium phosphate (n-ACP) were combined to synthesize a novel nanomaterial, carboxylated polyamidoamine dendrimer/amorphous calcium phosphate nanocomposite (PAMAM-COOH/ACP). This article aims to evaluate the remineralization effect of PAMAM-COOH/ACP of dentin type I collagen fibrils in vitro. Fluorescence labeling technique was innovatively used to observe and evaluate the remineralization effect. PAMAM-COOH/ACP showed superior remineralization ability of human dentin type I collagen fibrils, especially the intrafibrillar remineralization. Therefore, the novel nanomaterial PAMAM-COOH/ACP is promising to prevent and treat various diseases caused by dentin demineralization and to improve various dental materials.

c-Axis-Oriented Platelets of Crystalline Hydroxyapatite in Biomimetic Intrafibrillar Mineralization of Polydopamine-Functionalized Collagen Type I

Mineralized collagen fibrils are important basic building blocks of calcified tissues, such as bone and dentin. Polydopamine (PDA) can introduce functional groups, i.e., hydroxyl and amine groups, on the surfaces of type I collagen (Col-I) as possible nucleation sites of calcium phosphate (CaP) crystallization. Molecular bindings in between PDA and Col-I fibrils (Col-PDA) have been found to significantly reduce the interfacial energy. The wetting effect, mainly hydrophilicity due to the functional groups, escalates the degree of mineralization. The assembly of Col-I molecules into fibrils was initiated at the designated number of collagenous molecules and PDA. In contrast to the infiltration of amorphous calcium phosphate (ACP) precursors into the Col-I matrix by polyaspartic acid (pAsp), this collagen assembly process allows nucleation and ACP to exist in advance by PDA in the intrafibrillar matrix. PDA bound to specific sites, i.e., gap and overlap zones, by the regular arrangement of Col-I fibrils enhanced ACP nucleation and thus mineralization. As a result, the c-axis-oriented platelets of crystalline hydroxyapatite in the Col-I fibril matrix were observed in the enhanced mineralization through PDA functionalization.

Decrease of lysyl hydroxylase 2 activity causes abnormal collagen molecular phenotypes, defective mineralization and compromised mechanical properties of bone

Lysyl hydroxylase 2 (LH2) is an enzyme that catalyzes the hydroxylation of lysine (Lys) residues in fibrillar collagen telopeptides, a critical post-translational modification for the stability of intermolecular cross-links. Though abnormal LH2 activities have been implicated in various diseases including Bruck syndrome, the molecular basis of the pathologies is still not well understood. Since LH2 null mice die at early embryonic stage, we generated LH2 heterozygous (LH2^+/-) mice in which LH2 level is significantly diminished, and characterized collagen and bone phenotypes using femurs. Compared to the wild-type (WT), LH2^+/- collagen showed a significant decrease in the ratio of hydroxylysine (Hyl)- to the Lys-aldehyde-derived collagen cross-links without affecting the total number of aldehydes involved in cross-links. Mass spectrometric analysis revealed that, in LH2^+/- type I collagen, the extent of hydroxylation of all telopeptidyl Lys residues was significantly decreased. In the helical domain, Lys hydroxylation at the cross-linking sites was either unaffected or slightly lower, but other sites were significantly diminished compared to WT. In LH2^+/- femurs, mineral densities of cortical and cancellous bones were significantly decreased and the mechanical properties of cortical bones evaluated by nanoindentation analysis were compromised. When cultured, LH2^+/- osteoblasts poorly produced mineralized nodules compared to WT osteoblasts. These data provide insight into the functionality of LH2 in collagen molecular phenotype and its critical role in bone matrix mineralization and mechanical properties.

Oriented Crystallization of Hydroxyapatite in Self-Assembled Peptide Fibrils as a Bonelike Material

Controlling oriented crystallization is key to producing bonelike composite materials with a well-organized structure. However, producing this type of composite material using synthetic biopolymers as scaffolds is challenging. Inspired by the molecular structure of collagen-I, a collagenlike peptide─(Pro-Hyp-Gly)₁₀ (POG10)─was designed to produce self-assembled fibrils that resemble the structure of collagen-I fibrils. In addition, the oriented mineralization of HAP crystals is formed in the fibrils that reproduces a bonelike material similar to collagen-I fibril mineralization. Unlike collagen-I fibrils, POG10 fibrils do not contain gap spaces. The molecular simulation results indicate that in addition to space confinement, the molecular field generated by POG10 can also confine the orientation of HAP, enriching our understanding of physical confinement and shedding light on the design of synthetic biopolymer scaffolds for bonelike material fabrication.

Hierarchical ultrastructure: An overview of what is known about tendons and future perspective for tendon engineering

Abnormal tendons are rarely ever repaired to the natural structure and morphology of normal tendons. To better guide the repair and regeneration of injured tendons through a tissue engineering method, it is necessary to have insights into the internal morphology, organization, and composition of natural tendons. This review summarized recent researches on the structure and function of the extracellular matrix (ECM) components of tendons and highlight the application of multiple detection methodologies concerning the structure of ECMs. In addition, we look forward to the future of multi-dimensional biomaterial design methods and the potential of structural repair for tendon ECM components. In addition, focus is placed on the macro to micro detection methods for tendons, and current techniques for evaluating the extracellular matrix of tendons at the micro level are introduced in detail. Finally, emphasis is given to future extracellular matrix detection methods, as well as to how future efforts could concentrate on fabricating the biomimetic tendons.

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Summarize recent research on the structure and function of the extracellular matrix (ECM) components of tendons.

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Comments on current research methods concerning the structure of ECMs.

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Perspective on the future of multi-dimensional detection techniques and structural repair of tendon ECM components.

A Dopamine Acrylamide Molecule for Promoting Collagen Biomimetic Mineralization and Regulating Crystal Growth Direction

The reconstruction of the intra/interfibrillar mineralized collagen microstructure is extremely important in biomaterial science and regeneration medicine. However, certain problems, such as low efficiency and long period of mineralization, are apparent, and the mechanism of interfibrillar mineralization is often neglected in the present literature. Thus, we propose a novel model of biomimetic collagen mineralization that uses molecules with the dual function of cross-linking collagen and regulating collagen mineralization to construct the intrafibrillar and interfibrillar collagen mineralization of the structure of mineralized collagen hard tissues. In the present study completed in vitro, N-2-(3,4-dihydroxyphenyl) acrylamide (DAA) is used to bind and cross-link collagen molecules and further stabilize the self-assembled collagen fibers. The DAA-collagen complex provides more affinity with calcium and phosphate ions, which can reduce the calcium phosphate/collagen interfacial energy to promote hydroxyapatite (HA) nucleation and accelerate the rate of collagen fiber mineralization. Besides inducing intrafibrillar mineralization, the DAA-collagen complex mineralization template can realize interfibrillar mineralization with the c-axis of the HA crystal on the surface of collagen fibers and between fibers that are parallel to the long axis of collagen fibers. The DAA-collagen complex, as a new type of mineralization template, may provide a new collagen mineralization strategy to produce a mineralized scaffold material for tissue engineering or develop bone-like materials.

Modification of COL1A1 in Autologous Adipose Tissue-Derived Progenitor Cells Rescues the Bone Phenotype in a Mouse Model of Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is a congenital genetic disorder mainly manifested as bone fragility and recurrent fracture. Mutation of COL1A1/COL1A2 genes encoding the type I collagen are most responsible for the clinical patients. Allogenic mesenchymal stem cells (MSCs) provide the potential to treat OI through differentiation into osteoblasts. Autologous defective MSCs have not been utilized in OI treatment mainly because of their impaired osteogenesis, but the latent mechanism has not been well understood. Here, the relative signaling abnormality of adipose-derived mesenchymal stem cells (ADSCs) isolated from OI type I mice (Col1a1^+/-365 mice) was explored. Autologous ADSCs transfected by retrovirus carrying human COL1A1 gene was first utilized in OI therapy. The results showed that decreased activity of Yes-associated protein (YAP) due to hyperactive upstream Hippo kinases greatly contributed to the weakened bone-forming capacity of defective ADSCs. Recovered collagen synthesis of autologous ADSCs by COL1A1 gene modification normalized Hippo/YAP signaling and effectively rescued YAP-mediated osteogenesis. And the COL1A1 gene engineered autologous ADSCs efficaciously improved the microstructure, enhanced the mechanical properties and promoted bone formation of Col1a1^+/-365 mice after femoral bone marrow cavity delivery and might serve as an alternative source of stem cells in OI treatment. © 2021 American Society for Bone and Mineral Research (ASBMR).

Finite element analysis of the impact of bone nanostructure on its piezoelectric response

The piezoelectric response of bone at the submicron scale is analyzed under mechanical loadings using the finite element (FE) method. A new algorithm is presented to virtually reconstruct realistic bone nanostructures, consisting of collagen fibrils embedded in a hydroxyapatite mineral network. This algorithm takes into account potential misalignments between fibrils, as well the porous structure of the mineral phase. A parallel non-iterative mesh generation algorithm is utilized to create high-fidelity FE models for several representative volume elements (RVEs) of the bone with various fibrils volume fractions and misalignments. The piezoelectric response of each RVE is simulated under three types of loading: the longitudinal compression, lateral compression, and shear. The resulting homogenized stress and electric field in RVEs with aligned fibrils showed a linear variation with the fibrils volume fraction under all loading conditions. For RVEs with misaligned fibrils, although more oscillations were observed in homogenized results, their difference with the results of RVEs with aligned fibrils subject to lateral compression and shear loadings were negligible. However, under longitudinal compression, the electric field associated with RVEs with misaligned fibrils was notably higher than that of RVEs with aligned fibrils for the same volume fraction.

Osteocytes but not osteoblasts directly build mineralized bone structures

Bone-forming osteoblasts have been a cornerstone of bone biology for more than a century. Most research toward bone biology and bone diseases center on osteoblasts. Overlooked are the 90% of bone cells, called osteocytes. This study aims to test the hypothesis that osteocytes but not osteoblasts directly build mineralized bone structures, and that defects in osteocytes lead to the onset of hypophosphatemia rickets. The hypothesis was tested by developing and modifying multiple imaging techniques, including both in vivo and in vitro models plus two types of hypophosphatemia rickets models (Dmp1-null and Hyp, Phex mutation mice), and Dmp1-Cre induced high level of β-catenin models. Our key findings were that osteocytes (not osteoblasts) build bone similar to the construction of a high-rise building, with a wire mesh frame (i.e., osteocyte dendrites) and cement (mineral matrices secreted from osteocytes), which is a lengthy and slow process whose mineralization direction is from the inside toward the outside. When osteoblasts fail to differentiate into osteocytes but remain highly active in Dmp-1-null or Hyp mice, aberrant and poor bone mineralization occurs, caused by a sharp increase in Wnt-β-catenin signaling. Further, the constitutive expression of β-catenin in osteocytes recaptures a similar osteomalacia phenotype as shown in Dmp1 null or Hyp mice. Thus, we conclude that osteocytes directly build bone, and osteoblasts with a short life span serve as a precursor to osteocytes, which challenges the existing dogma.

The Cell Origin and Role of Osteoclastogenesis and Osteoblastogenesis in Vascular Calcification

Arterial calcification refers to the abnormal deposition of calcium salts in the arterial wall, which results in vessel lumen stenosis and vascular remodeling. Studies increasingly show that arterial calcification is a cell mediated, reversible and active regulated process similar to physiological bone mineralization. The osteoblasts and chondrocytes-like cells are present in large numbers in the calcified lesions, and express osteogenic transcription factor and bone matrix proteins that are known to initiate and promote arterial calcification. In addition, osteoclast-like cells have also been detected in calcified arterial walls wherein they possibly inhibit vascular calcification, similar to the catabolic process of bone mineral resorption. Therefore, tilting the balance between osteoblast-like and osteoclast-like cells to the latter maybe a promising therapeutic strategy against vascular calcification. In this review, we have summarized the current findings on the origin and functions of osteoblast-like and osteoclast-like cells in the development and progression of vascular progression, and explored novel therapeutic possibilities.

Biomineralization of Collagen-Based Materials for Hard Tissue Repair

Hydroxyapatite (HA) reinforced collagen fibrils serve as the basic building blocks of natural bone and dentin. Mineralization of collagen fibrils play an essential role in ensuring the structural and mechanical functionalities of hard tissues such as bone and dentin. Biomineralization of collagen can be divided into intrafibrillar and extrafibrillar mineralization in terms of HA distribution relative to collagen fibrils. Intrafibrillar mineralization is termed when HA minerals are incorporated within the gap zone of collagen fibrils, while extrafibrillar mineralization refers to the minerals that are formed on the surface of collagen fibrils. However, the mechanisms resulting in these two types of mineralization still remain debatable. In this review, the evolution of both classical and non-classical biomineralization theories is summarized. Different intrafibrillar mineralization mechanisms, including polymer induced liquid precursor (PILP), capillary action, electrostatic attraction, size exclusion, Gibbs-Donnan equilibrium, and interfacial energy guided theories, are discussed. Exemplary strategies to induce biomimetic intrafibrillar mineralization using non-collagenous proteins (NCPs), polymer analogs, small molecules, and fluidic shear stress are discussed, and recent applications of mineralized collagen fibers for bone regeneration and dentin repair are included. Finally, conclusions are drawn on these proposed mechanisms, and the future trend of collagen-based materials for bone regeneration and tooth repair is speculated.

Involvement of prenucleation clusters in calcium phosphate mineralization of collagen

Involvement of thermodynamically-stable prenucleation clusters (PNCs) in the biomineralization of collagen has been speculated since their existence was reported in mineralization systems. It has been hypothesized that intrafibrillar mineralization proceeds via nucleation of inhibitor-stabilized intermediates produced by liquid-liquid separation (aka. polymer-induced liquid precursors; PILPs). Here, the contribution of PNCs and PILPs to calcium phosphate intrafibrillar mineralization of collagen was examined in a model with a semipermeable membrane that excludes nucleation inhibitor-stabilized PILPs from reaching the collagen fibrils, using cryogenic electron microscopy of reconstituted fibrils and conventional transmission electron microscopy of collagen sponges. Molecular dynamics simulation with the Interface force field (IFF) was used to confirm the existence of PILPs with amorphous calcium phosphate and elucidate details of the dynamics. Furthermore, intrafibrillar mineralization of single collagen fibrils was experimentally observed with unstabilized PNCs when anionic/cationic polyelectrolytes were used to establish Donnan equilibrium across the semipermeable membrane. Molecular dynamics simulation verified PNC formation within the collagen intrafibrillar gap zones at the atomic scale and explained the role of external PILPs. The PILPs decrease the interfibrillar water content and increase the interfibrillar ionic concentration. Nevertheless, intrafibrillar mineralization of collagen sponges with PNCs alone was inefficacious, being constrained by competition from extrafibrillar mineral precipitation. STATEMENT OF SIGNIFICANCE: Compared with conventional PILP-based intrafibrillar mineralization, mineralization of collagen fibrils using unstabilized PNCs is constrained by competition from extrafibrillar mineral deposition. The narrow window of opportunity for PNCs to produce intrafibrillar mineralization provides a plausible explanation for the feasibility of nucleation inhibitor-free intrafibrillar apatite assembly during reconstitution of type I collagen.

Polyelectrolyte-calcium complexes as a pre-precursor induce biomimetic mineralization of collagen

Polyelectrolytes such as polyaspartic acid (PAsp) are critical in biomimetic mineralization as stabilizers of amorphous calcium phosphate (ACP) precursors and as nucleation inhibitors similar to non-collagenous proteins (NCPs). Nevertheless, the application of polyelectrolyte-calcium complexes as a pre-precursor, such as PAsp-Ca complexes, in the mineralization of collagen is unexplored. Herein, we propose a polyelectrolyte-Ca complex pre-precursor (PCCP) process for collagen mineralization. By combining three-dimensional (3D) STORM, potential measurements, and cryogenic transmission electron microscopy with molecular dynamics simulations, we show that liquid-like electropositive PAsp-Ca complexes along with free calcium ions infiltrate electronegative collagen fibrils. The PAsp-Ca complexes are immobilized within the fibrils via chelation and hydrogen bonds, and outward movement of free calcium ions is prevented while phosphate and hydroxide are recruited through electrostatic attractions. Afterwards, ACP instantly forms and gradually crystallizes. The PCCP process not only unites two distinct crystallization pathways (classical (free Ca/P ions) and non-classical (polyelectrolyte-Ca complexes)), but also provides a novel strategy for rapid biomimetic mineralization of collagen.

Collagen Structure-Function Mapping Informs Applications for Regenerative Medicine

Type I collagen, the predominant protein of vertebrates, assembles into fibrils that orchestrate the form and function of bone, tendon, skin, and other tissues. Collagen plays roles in hemostasis, wound healing, angiogenesis, and biomineralization, and its dysfunction contributes to fibrosis, atherosclerosis, cancer metastasis, and brittle bone disease. To elucidate the type I collagen structure-function relationship, we constructed a type I collagen fibril interactome, including its functional sites and disease-associated mutations. When projected onto an X-ray diffraction model of the native collagen microfibril, data revealed a matrix interaction domain that assumes structural roles including collagen assembly, crosslinking, proteoglycan (PG) binding, and mineralization, and the cell interaction domain supporting dynamic aspects of collagen biology such as hemostasis, tissue remodeling, and cell adhesion. Our type III collagen interactome corroborates this model. We propose that in quiescent tissues, the fibril projects a structural face; however, tissue injury releases blood into the collagenous stroma, triggering exposure of the fibrils' cell and ligand binding sites crucial for tissue remodeling and regeneration. Applications of our research include discovery of anti-fibrotic antibodies and elucidating their interactions with collagen, and using insights from our angiogenesis studies and collagen structure-function model to inform the design of super-angiogenic collagens and collagen mimetics.

Piezoelectric Heterogeneity in Collagen Type I Fibrils Quantitatively Characterized by Piezoresponse Force Microscopy

Piezoelectricity of Type I collagen can provide the stress-generated potential that is considered to be one of the candidate mechanisms to explain bone's adaptation to loading. However, it is still challenging to quantify piezoelectricity because of its heterogeneity and small magnitude. In this study, resonance-enhanced piezoresponse force microscopy (PFM) was utilized to amplify a weak piezoresponse of a single collagen fibril with a carefully calibrated cantilever. The quantitative PFM, combined with a dual-frequency resonance-tracking method, successfully identified the anisotropic and heterogenous nature of the piezoelectric properties in the collagen fibril. The profile of shear piezoelectric coefficient (d₁₅) was obtained to be periodic along the collagen fibril, with a larger value in the gap zone (0.51 pm/V) compared to the value in the overlap zone (0.29 pm/V). Interestingly, this piezoelectric profile corresponds to the periodic profile of mechanical stiffness in a mineralized collagen fibril having a higher stiffness in the gap zone. Considering that apatite crystals are nucleated at the gap zone and subsequently grown along the collagen fibril, the heterogeneous and anisotropic nature of piezoelectric properties highlights the physiological importance of the collagen piezoelectricity in bone mineralization.

L-histidine controls the hydroxyapatite mineralization with plate-like morphology: Effect of concentration and media

Hydroxyapatite (HA) is the main inorganic component of bone and dentin, and their non-stoichiometric compositions and plate-shaped morphology is responsible for their bioactivity and osteoconductive nature. Collagenous (CPs) and non-collagenous proteins (NCPs) facilitate mineralization and regulate structural properties of HA through their side-chains. The bioactivity of synthetic HA does not usually match with the HA found in bone and, therefore, there is a need to understand the role of biomolecules in bone mineralization in order to develop non-stoichiometric plate-shaped HA for bone grafts. Role of several amino acids has been investigated but the role of L-his has been rarely investigated under physiological conditions even though it is a part of HA inhibitor proteins, like albumin, amelogenin, and histidine-rich proteins. In this study, L-his and L-glu were used to modify the structural properties of HA in different experimental conditions and buffer systems (tris and hepes). The results showed that L-his was able to regulate the plate-shaped morphology of HA in every experimental condition, unlike the L-glu, where the crystal morphology was regulated by experimental conditions. Both amino acids behaved differently in DI water, tris, and hepes buffer, and the media used influenced the precipitation time and structural properties of HA. Hepes and tris buffers also influenced the HA precipitation process. Overall, the studies revealed that L-his may be used as an effective regulator of plate-shaped morphology of HA, instead of large NCPs/proteins, for designing biomaterials for bone regeneration applications and the choice of buffer system is important in designing and evaluating the systems for mineralization. In cell culture studies, mouse osteoblast precursor cells (MC3T3-E1) showed highest proliferation on the bone-like plate-shaped HA, among all the HA samples investigated.

Nanoparticle-directed and ionically forced polyphosphate coacervation: a versatile and reversible core-shell system for drug delivery

A drug encapsulation/delivery system using a novel principle is described that is based on an intra-particle migration of calcium ions between a central Ca2+-enriched nanoparticle core and the surrounding shell compartment. The supply of Ca2+ is needed for the formation of a coacervate shell around the nanoparticles, acting as the core of drug-loadable core-shell particles, using the physiological inorganic polymer polyphosphate (polyP). This polyanion has the unique property to form, at an alkaline pH and in the presence of a stoichiometric surplus of calcium ions, water-insoluble and stabile amorphous nanoparticles. At neutral pH a coacervate, the biologically active form of the polymer, is obtained that is composed of polyP and Ca2+. The drug-loaded core-shell particles, built from the Ca-polyP core and the surrounding Ca-polyP shell, were fabricated in two successive steps. First, the formation of the nanoparticle core at pH 10 and a superstoichiometric 2:1 molar ratio between CaCl2 and Na-polyP into which dexamethasone, as a phosphate derivative, was incorporated. Second, the preparation of the coacervate shell, loaded with ascorbic acid, by exposure of the Ca-polyP core to soluble Na-polyP and L-ascorbate (calcium salt). EDX analysis revealed that during this step the Ca2+ ions required for coacervate formation migrate from the Ca-polyP core (with a high Ca:P ratio) to the shell. Electron microscopy of the particles show an electron-dense 150-200 nm sized core surrounded by a less sharply delimited electron-sparse shell. The core-shell particles exhibited strong osteogenic activity in vitro, based on the combined action of polyP and of dexamethasone and ascorbic acid, which reversibly bind to the anionic polyP via ionic Ca2+ bonds. Drug release from the particles occurs after contact with a peptide/protein-containing serum, a process which is almost complete after 10 days and accompanied by the conversion of the nanoparticles into a coacervate. Human osteosarcoma SaOS-2 cells cultivated onto or within an alginate hydrogel matrix showed increased growth/viability and mineralization when the hybrid particles containing dexamethasone and ascorbic acid were embedded in the matrix. The polyP-based core-shell particles have the potential to become a suitable, pH-responsive drug encapsulation/release system, especially for bone, cartilage and wound healing.

Citrate regulates extracellular matrix mineralization during osteoblast differentiation in vitro

The extremely high levels of citrate in bone highlight its important role, which must be involved in some essential functional or structural role that is required for the development and maintenance of normal bone. However, biomineralization researches have emphasized the interaction between the citrate and inorganic minerals during crystallization in cell-free systems. It is difficult to obtain a thorough and comprehensive understanding from cell-free experimental conditions and treatment methods. In this study, by proposing an osteoblast mineralization experimental model, we explored the regulation of citrate on bone apatite crystal structure. Our studies show that citrate stabilizes two precursors and then inhibits their transformation into hydroxyapatite. Concomitantly, the smaller size and lower crystallinity mineral deposition emerge during citrate-mediated osteogenic mineralization. These findings may provide a new perspective for the mechanism of osteogenic mineralization and a basis for further understanding of bone metabolism.

Intermolecular channels direct crystal orientation in mineralized collagen

The mineralized collagen fibril is the basic building block of bone, and is commonly pictured as a parallel array of ultrathin carbonated hydroxyapatite (HAp) platelets distributed throughout the collagen. This orientation is often attributed to an epitaxial relationship between the HAp and collagen molecules inside 2D voids within the fibril. Although recent studies have questioned this model, the structural relationship between the collagen matrix and HAp, and the mechanisms by which collagen directs mineralization remain unclear. Here, we use XRD to reveal that the voids in the collagen are in fact cylindrical pores with diameters of ~2 nm, while electron microscopy shows that the HAp crystals in bone are only uniaxially oriented with respect to the collagen. From in vitro mineralization studies with HAp, CaCO₃ and γ-FeOOH we conclude that confinement within these pores, together with the anisotropic growth of HAp, dictates the orientation of HAp crystals within the collagen fibril.

IFT20 is critical for collagen biosynthesis in craniofacial bone formation

Intraflagellar transport (IFT) is essential for assembling primary cilia required for bone formation. Disruption of IFT frequently leads to bone defects in humans. While it has been well studied about the function of IFT in osteogenic cell proliferation and differentiation, little is known about its role in collagen biosynthesis during bone formation. Here we show that IFT20, the smallest IFT protein in the IFT-B complex, is important for collagen biosynthesis in mice. Deletion of Ift20 in craniofacial osteoblasts displayed bone defects in the face. While collagen protein levels are unaffected by loss of Ift20, collagen cross-linking was significantly altered. In both Ift20:Wnt1-Cre and Ift20:Ocn-Cre mice the bones exhibit increased hydroxylysine-aldehyde deived cross-linking, and decreased lysine-aldehyde derived cross-linking. To obtain insight into the molecular mechanisms, we examined the expression levels of telopeptidyl lysyl hydroxylase 2 (LH2), and associated chaperone complexes. The results demonstrated that, while LH2 levels were unaffected by loss of Ift20, its chaperone, FKBP65, was significantly increased in Ift20:Wnt1-Cre and Ift20:Ocn-Cre mouse calvaria as well as femurs. These results suggest that IFT20 plays a pivotal role in collagen biosynthesis by regulating, in part, telopeptidyl lysine hydroxylation and cross-linking in bone. To the best of our knowledge, this is the first to demonstrate that the IFT components control collagen post-translational modifications. This provides a novel insight into the craniofacial bone defects associated with craniofacial skeletal ciliopathies.

Structural Biology of Calcium Phosphate Nanoclusters Sequestered by Phosphoproteins

Biofluids that contain stable calcium phosphate nanoclusters sequestered by phosphopeptides make it possible for soft and hard tissues to co-exist in the same organism with relative ease. The stability diagram of a solution of nanocluster complexes shows how the minimum concentration of phosphopeptide needed for stability increases with pH. In the stable region, amorphous calcium phosphate cannot precipitate. Nevertheless, if the solution is brought into contact with hydroxyapatite, the crystalline phase will grow at the expense of the nanocluster complexes. The physico-chemical principles governing the formation, composition, size, structure, and stability of the complexes are described. Examples are given of complexes formed by casein, osteopontin, and recombinant phosphopeptides. Application of these principles and properties to blood serum, milk, urine, and resting saliva is described to show that under physiological conditions they are in the stable region of their stability diagram and so cannot cause soft tissue calcification. Stimulated saliva, however, is in the metastable region, consistent with its role in tooth remineralization. Destabilization of biofluids, with consequential ill-effects, can occur when there is a failure of homeostasis, such as an increase in pH without a balancing increase in the concentration of sequestering phosphopeptides.

Three-dimensional structural interrelations between cells, extracellular matrix, and mineral in normally mineralizing avian leg tendon

The spatial-temporal relationship between cells, extracellular matrices, and mineral deposits is fundamental for an improved understanding of mineralization mechanisms in vertebrate tissues. By utilizing focused ion beam-scanning electron microscopy with serial surface imaging, normally mineralizing avian tendons have been studied with nanometer resolution in three dimensions with volumes exceeding tens of micrometers in range. These parameters are necessary to yield sufficiently fine ultrastructural details while providing a comprehensive overview of the interrelationships between the tissue structural constituents. Investigation reveals a complex lacuno-canalicular network in highly mineralized tendon regions, where ∼100 nm diameter canaliculi emanating from cell (tenocyte) lacunae surround extracellular collagen fibril bundles. Canaliculi are linked to smaller channels of ∼40 nm diameter, occupying spaces between fibrils. Close to the tendon mineralization front, calcium-rich deposits appear between the fibrils and, with time, mineral propagates along and within them. These close associations between tenocytes, tenocyte lacunae, canaliculi, small channels, collagen, and mineral suggest a concept for the mineralization process, where ions and/or mineral precursors may be transported through spaces between fibrils before they crystallize along the surface of and within the fibrils.

Promotion effect of immobilized chondroitin sulfate on intrafibrillar mineralization of collagen

Chondroitin sulfate (CS) is widespread in mineralized tissues and is considered to play crucial roles during the mineralization process. However, its role in biomineralization remains controversial. In the present study, CS is immobilized to collagen fibrils to mimic its state in biomineralization. The results demonstrate that immobilized CS on collagen fibrils accelerates calcium phosphate nucleation and significantly promotes collagen mineralization by accumulating calcium ions in collagen fibrils. The stochastic optical reconstruction microscopy results confirm that CS gives the specific nucleation sites for calcium phosphate to preferentially form, the improved intrafibrillar heterogeneous nucleation of calcium phosphate facilitates intrafibrillar mineralization. It is found remarkably accelerated remineralization of CS immobilized demineralized dentin is achieved. This study offers insight on the understanding of the function of the biomacromolecule CS on the biomineralization front. In addition, CS effectively promotes intrafibrillar mineralization, which highlights fine prospect for CS to reconstruct collagen-mineralized tissues as a natural material.

Applications of Carbon Nanotubes in Bone Tissue Regeneration and Engineering: Superiority, Concerns, Current Advancements, and Prospects

With advances in bone tissue regeneration and engineering technology, various biomaterials as artificial bone substitutes have been widely developed and innovated for the treatment of bone defects or diseases. However, there are no available natural and synthetic biomaterials replicating the natural bone structure and properties under physiological conditions. The characteristic properties of carbon nanotubes (CNTs) make them an ideal candidate for developing innovative biomimetic materials in the bone biomedical field. Indeed, CNT-based materials and their composites possess the promising potential to revolutionize the design and integration of bone scaffolds or implants, as well as drug therapeutic systems. This review summarizes the unique physicochemical and biomedical properties of CNTs as structural biomaterials and reinforcing agents for bone repair as well as provides coverage of recent concerns and advancements in CNT-based materials and composites for bone tissue regeneration and engineering. Moreover, this review discusses the research progress in the design and development of novel CNT-based delivery systems in the field of bone tissue engineering.

Biomineralization of dentin

A single biomineralization of demineralized dentin is significant to restore the demineralized dentin due to dental caries or erosion. In recent years, meaningful progress has been made regarding the mechanisms involved in the biomineralization of dentin collagen. Concepts changing from the classical ion-based crystallization to non-classical particle-based crystallization, inspired a different strategy to infiltrate the demineralized dentin collagen. The remarkable discovery was the report of liquid-like amorphous calcium phosphate as nanoprecursor particles to carbonated hydroxyapatite. The non-collagenous proteins and their analogues are widely investigated, for their key role in controlling mineralization during the process of crystal nucleation and growth. The in-depth studies of the gap zone provided significant improvements in our understanding of the structure of collagen and of the intrafibrillar remineralization of collagen fibrils. The collagen is not a passive substrate as previously supposed, and the active role of guiding nanoprecursor infiltration and mediating its nucleation has been demonstrated. Furthermore, recovery of mechanical properties has been evaluated to determine the effectiveness of dentin remineralization. Finally, the problems regarding the origin formation of the calcium phosphate that is deposited in the collagen, and the exact interactions between the non-collagenous proteins, amorphous calcium phosphate and collagen are still unclear. We reviewed the importance of these findings in enriching our understanding of dentin biomineralization, while addressing certain limitations that are inherent to in vitro studies.

An in vitro study on dentin demineralization and remineralization: Collagen rearrangements and influence on the enucleated phase

Dentin remineralization is of clinical relevance in the therapy of caries and dentin hypersensitivity. This study is aimed at gaining more insights on a molecular scale, through IR spectroscopy, into dentin demineralization and remineralization. The dentin demineralization by ethylenediaminetetraacetic acid, EDTA (17%, 2 h) significantly altered the secondary structure distribution of collagen, upon loss of interaction with calcium ions. To investigate dentin remineralization, previously demineralized human dentin slices were soaked in Dulbecco's Phosphate Buffered Saline (DPBS) or Hank's Balanced Salt Solution HBSS, in close contact with three commercial cements used as sustained releasing sources of Ca²⁺ and OH^- ions (i.e. calcium hydroxide- and calcium silicate-based cements). IR spectroscopy showed the occurrence of remineralization under these conditions. Collagen did not lose its ability to chelate Ca²⁺, and these interactions allowed collagen to rearrange into a conformation similar to that of sound dentin. This process appeared slower in HBSS than DPBS, as also shown by the lower degree of maturation of the inorganic phase enucleated in the former medium (amorphous calcium phosphate versus B-type carbonated apatite). Collagen appeared to act as a spatial constraint to crystal deposition, affecting crystallinity and carbonate content of the enucleated phase. Remineralization was found to strongly depend on the calcium releasing ability of the cements. The fast formation of a rough apatite biocoating may represent a favorable clinical condition in the context of mineralized tissue regeneration.

Phosphorylated chitosan to promote biomimetic mineralization of type I collagen as a strategy for dentin repair and bone tissue engineering

This novel biomimetic mineralization technique provides an efficient method to produce an advanced mineralized matrix.

Organization of Bone Mineral: The Role of Mineral–Water Interactions

The mechanism (s) that drive the organization of bone mineral throughout the bone extracellular matrix remain unclear. The long-standing theory implicates the organic matrix, namely specific non-collagenous proteins and/or collagen fibrils, while a recent theory proposes a self-assembly mechanism. Applying a combination of spectroscopic and microscopic techniques in wet and dry conditions to bone-like hydroxyapatite nanoparticles that were used as a proxy for bone mineral, we confirm that mature bone mineral particles have the capacity to self-assemble into organized structures. A large quantity of water is present at the surface of bone mineral due to the presence of a hydrophilic, amorphous surface layer that coats bone mineral nanoparticles. These water molecules must not only be strongly bound to the surface of bone mineral in the form of a rigid hydration shell, but they must also be trapped within the amorphous surface layer. Cohesive forces between these water molecules present at the mineral–mineral interface not only hold the mature bone mineral particles together, but also promote their oriented stacking. This intrinsic ability of mature bone mineral particles to organize themselves without recourse to the organic matrix forms the foundation for the development of the next generation of orthopedic biomaterials.

Comparison of the volumetric composition of lamellar bone and the woven bone of calluses

Woven tissue is mainly present in the bone callus, formed very rapidly either after a fracture or in distraction processes. This high formation speed is probably responsible for its disorganized microstructure and this, in turn, for its low stiffness. Nonetheless, the singular volumetric composition of this tissue may also play a key role in its mechanical properties. The volumetric composition of woven tissue extracted from the bone transport callus of sheep was investigated and compared with that of the lamellar tissue extracted from the cortical shell of the same bone. Significant differences were found in the mineral and water contents, but they can be due to the different ages of both tissues, which affects the mineral/water ratio. However, the content in organic phase remains more or less constant throughout the mineralization process and has proven to be a good variable to measure the different composition of both tissues, being that content significantly higher in woven tissue. This may be linked to the abnormally high concentration of osteocytes in this tissue, which is likely a consequence of the more abundant presence of osteoblasts secreting osteoid and burying other osteoblasts, which then differentiate into osteocytes. This would explain the high formation rate of woven tissue, useful to recover the short-term stability of the bone. Nonetheless, the more abundant presence of organic phase prevents the woven tissue from reaching a stiffness similar to that of lamellar tissue in the long term, when it is fully mineralized.

Support of bone mineral deposition by regulation of pH

Osteoblasts secrete collagen and isolate bone matrix from extracellular space. In the matrix, alkaline phosphatase generates phosphate that combines with calcium to form mineral, liberating 8 H⁺ per 10 Ca⁺² deposited. However, pH-dependent hydroxyapatite deposition on bone collagen had not been shown. We studied the dependency of hydroxyapatite deposition on type I collagen on pH and phosphate by surface plasmon resonance in 0-5 mM phosphate at pH 6.8-7.4. Mineral deposition saturated at <1 mM Ca²⁺ but was sensitive to phosphate. Mineral deposition was reversible, consistent with amorphous precipitation; stable deposition requiring EDTA removal appeared with time. At pH 6.8, little hydroxyapatite deposited on collagen; mineral accumulation increased 10-fold at pH 7.4. Previously, we showed high expression Na⁺/H⁺ exchanger (NHE) and ClC transporters in osteoblasts. We hypothesized that, in combination, these move protons across osteoblasts to the general extracellular space. We made osteoblast membrane vesicles by nitrogen cavitation and used acridine orange quenching to characterize proton transport. We found H⁺ transport dependent on gradients of chloride or sodium, consistent with apical osteoblast ClC family Cl^-,H⁺ antiporters and basolateral osteoblast NHE family Na⁺/H⁺ exchangers. Little, if any, active H⁺ transport, supported by ATP, occurred. Major transporters include cariporide-sensitive NHE1 in basolateral membranes and ClC3 and ClC5 in apical osteoblast membranes. The mineralization inhibitor levamisole reduced bone formation and expression of alkaline phosphatase, NHE1, and ClC5. We conclude that mineral deposition in bone collagen is pH-dependent, in keeping with H⁺ removal by Cl^-,H⁺ antiporters and Na⁺/H⁺-exchangers. Periodic orientation hydroxyapatite is organized on type I collagen-coiled coils.

Osteocalcin facilitates calcium phosphate ion complex growth as revealed by free energy calculation

The nanoscopic structural and thermodynamic basis of biomolecule-regulated assembly and crystallization of inorganic solids have a tremendous impact on the rational design of novel functional nanomaterials, but are concealed by many difficulties in molecular-level characterization. Here we demonstrate that the free energy calculation approach, enabled by combining advanced molecular simulation techniques, can unravel the structural and energetic mechanisms of protein-mediated inorganic solid nucleation. It is observed that osteocalcin (OCN), an important non-collagenous protein involved in regulating bone formation, promotes the growth of nanosized calcium phosphate (CaP) ion clusters from a supersaturated solution. Free energy calculation by umbrella sampling indicates that this effect by OCN is prominent at the scale of 1 to 3 nm ion-association complexes (IACs). The binding interactions between gamma-carboxyl glutamate and the C-terminal and, interestingly, the arginine side chains of OCN and IACs stabilize under-coordinated IACs, thus promoting their growth. The promoter effect of OCN on the enlargement and further aggregation of IACs into cluster assemblies of tens of nm are confirmed by conventional molecular dynamics simulation and dynamic light scattering experiments. To the best of our knowledge, this is the first time that the free energy landscape of the early stages of CaP nucleation is shown. The free energy change as a function of IAC size shares the feature of decreasing monotonically as shown previously for the calcium carbonate system. Therefore, the nucleation of both these major biominerals apparently involves an initial phase of liquid-like ionic aggregates. The structural and thermodynamic information regarding OCN-CaP interactions amplifies the current understanding of biomineralization mechanisms at the nanoscale, with general relevance to biomolecule-tuned fabrication of inorganic materials.