Disordered Conformation with Low Pii Helix in Phosphoproteins Orchestrates Biomimetic Apatite Formation

Intrafibrillar Mineralization and Immunomodulatory for Synergetic Enhancement of Bone Regeneration via Calcium Phosphate Nanocluster Scaffold

Inspired by the bionic mineralization theory, organic-inorganic composites with hydroxyapatite nanorods orderly arranged along collagen fibrils have attracted extensive attention. Planted with an ideal bone scaffold will contribute greatly to the osteogenic microenvironment; however, it remains challenging to develop a biomimetic scaffold with the ability to promote intrafibrillar mineralization and simultaneous regulation of immune microenvironment in situ. To overcome these challenges, a scaffold containing ultra-small particle size calcium phosphate nanocluster (UsCCP) is prepared, which can enhance bone regeneration through the synergetic effect of intrafibrillar mineralization and immunomodulatory. By efficient infiltration into collagen fibrils, the UsCCP released from the scaffold achieves intrafibrillar mineralization. It also promotes the M2-type polarization of macrophages, leading to an immune microenvironment with both osteogenic and angiogenic potential. The results confirm that the UsCCP scaffold has both intrafibrillar mineralization and immunomodulatory effects, making it a promising candidate for bone regeneration.

Reversible, β-sheet-dependent self-assembly of the phosphoprotein phosvitin is controlled by the concentration and valency of cations

The hyperphosphorylated protein phosvitin (PV) undergoes a pH-dependent transition between P_II^- and β-sheet secondary structures, a process deemed crucial for its role in the promotion of biogenic apatite formation. The transition occurs surprisingly slowly (minutes to hours). This is consistent with a slow aggregation process involving ionic interactions of charged groups on the protein surface. Herein, we determined the associated transition pK values and time constants through matrix least-squares (MLS) global fitting of a series of pH- and time-dependent circular dichroism (CD) spectra recorded in the presence of different mono-, bi- and trivalent cations. Supporting our results with dynamic light scattering data, we clearly identified a close correlation of β-sheet transition and the formation of small aggregates at low pH. This process is inhibited in the presence of all tested cations with the strongest effects for trivalent cations (Fe³⁺ and Al³⁺). In the presence of Ca²⁺ and Mg²⁺, larger higher-order particles are formed from PV in the β-sheet conformation, as identified from the interpretation of differential scattering observed in the CD spectra. Our observations are consistent with the existence of a multi-step equilibrium between aggregated and non-aggregated species of PV. The equilibrium is highly sensitive to the environment pH and salt concentration with exceptional behavior in the presence of divalent cations such as Ca²⁺ and Mg²⁺.

Dynamic Structural Changes and Thermodynamics in Phase Separation Processes of an Intrinsically Disordered-Ordered Protein Model

Elastin-like proteins (ELPs) are biologically important proteins and models for intrinsically disordered proteins (IDPs) and dynamic structural transitions associated with coacervates and liquid-liquid phase transitions. However, the conformational status below and above coacervation temperature and its role in the phase separation process is still elusive. Employing matrix least-squares global Boltzmann fitting of the circular dichroism spectra of the ELPs (VPGVG)20 , (VPGVG)40 , and (VPGVG)60 , we found that coacervation occurs sharply when a certain number of repeat units has acquired β-turn conformation (in our sequence setting a threshold of approx. 20 repeat units). The character of the differential scattering of the coacervate suspensions indicated that this fraction of β-turn structure is still retained after polypeptide assembly. Such conformational thresholds may also have a role in other protein assembly processes with implications for the design of protein-based smart materials.

Collagen Suprafibrillar Confinement Drives the Activity of Acidic Calcium-Binding Polymers on Apatite Mineralization

Bone collagenous extracellular matrix provides a confined environment into which apatite crystals form. This biomineralization process is related to a cascade of events partly controlled by noncollagenous proteins. Although overlooked in bone models, concentration and physical environment influence their activities. Here, we show that collagen suprafibrillar confinement in bone comprising intra- and interfibrillar spaces drives the activity of biomimetic acidic calcium-binding polymers on apatite mineralization. The difference in mineralization between an entrapping dentin matrix protein-1 (DMP1) recombinant peptide (rpDMP1) and the synthetic polyaspartate validates the specificity of the 57-KD fragment of DMP1 in the regulation of mineralization, but strikingly without phosphorylation. We show that all the identified functions of rpDMP1 are dedicated to preclude pathological mineralization. Interestingly, transient apatite phases are only found using a high nonphysiological concentration of additives. The possibility to combine biomimetic concentration of both collagen and additives ensures specific chemical interactions and offers perspectives for understanding the role of bone components in mineralization.

The Thermodynamics of Medial Vascular Calcification

Medial vascular calcification (MVC) is a degenerative process that involves the deposition of calcium in the arteries, with a high prevalence in chronic kidney disease (CKD), diabetes, and aging. Calcification is the process of precipitation largely of calcium phosphate, governed by the laws of thermodynamics that should be acknowledged in studies of this disease. Amorphous calcium phosphate (ACP) is the key constituent of early calcifications, mainly composed of Ca2+ and PO4 3- ions, which over time transform into hydroxyapatite (HAP) crystals. The supersaturation of ACP related to Ca2+ and PO4 3- activities establishes the risk of MVC, which can be modulated by the presence of promoter and inhibitor biomolecules. According to the thermodynamic parameters, the process of MVC implies: (i) an increase in Ca2+ and PO4 3- activities (rather than concentrations) exceeding the solubility product at the precipitating sites in the media; (ii) focally impaired equilibrium between promoter and inhibitor biomolecules; and (iii) the progression of HAP crystallization associated with nominal irreversibility of the process, even when the levels of Ca2+ and PO4 3- ions return to normal. Thus, physical-chemical processes in the media are fundamental to understanding MVC and represent the most critical factor for treatments' considerations. Any pathogenetical proposal must therefore comply with the laws of thermodynamics and their expression within the medial layer.

Muscle-like Ultratough Hybrid Hydrogel Constructed by Heterogeneous Inorganic Polymerization on an Organic Network

Inspired by inorganic oligomers and their polymerization, we herein develop a heterogeneous inorganic polymerization tactic that can be used to prepare a muscle-like hybrid hydrogel by inducing the polymerization of calcium phosphate oligomers (CPO) onto a polyvinyl alcohol (PVA) molecular chain network. In this heterogeneous process, the CPO units bond with PVA molecules via assistance from sodium alginate (SA), and then gradually polymerize along the organic chains to form ultrafine hydroxyapatite nanolines with a diameter of ∼1 nm. Because of the well integration of organic and inorganic phases from the heterogeneous polymerization, the hierarchical structured hydrogel can exhibit ultratough mechanical properties of ∼17.84 MPa in strength and ∼8.97 kJ m^-2 in fracture energy, which exceed natural muscles and almost synthetic hydrogels. Moreover, the damaged hydrogel can be repaired readily by adding the precursors of CPO, PVA, and SA, which can induce in situ re-polymerization. The hydrogel also exhibits muscle-like rotational motion under aqueous conditions, which can be developed into a water-driven biomimetic motor. This study indicates that inorganic polymerization can achieve a novel organic-inorganic integration rather than conventional organic-inorganic composition, and it provides a novel tactic for design and manufacture of advanced materials.

Comparative Gene Analysis Focused on Silica Cell Wall Formation: Identification of Diatom-Specific SET Domain Protein Methyltransferases

Silica cell walls of diatoms have attracted attention as a source of nanostructured functional materials and have immense potential for a variety of applications. Previous studies of silica cell wall formation have identified numerous involved proteins, but most of these proteins are species-specific and are not conserved among diatoms. However, because the basic process of diatom cell wall formation is common to all diatom species, ubiquitous proteins and molecules will reveal the mechanisms of cell wall formation. In this study, we assembled de novo transcriptomes of three diatom species, Nitzschia palea, Achnanthes kuwaitensis, and Pseudoleyanella lunata, and compared protein-coding genes of five genome-sequenced diatom species. These analyses revealed a number of diatom-specific genes that encode putative endoplasmic reticulum-targeting proteins. Significant numbers of these proteins showed homology to silicanin-1, which is a conserved diatom protein that reportedly contributes to cell wall formation. These proteins also included a previously unrecognized SET domain protein methyltransferase family that may regulate functions of cell wall formation-related proteins and long-chain polyamines. Proteomic analysis of cell wall-associated proteins in N. palea identified a protein that is also encoded by one of the diatom-specific genes. Expression analysis showed that candidate genes were upregulated in response to silicon, suggesting that these genes play roles in silica cell wall formation. These candidate genes can facilitate further investigations of silica cell wall formation in diatoms.

Antibacterial and hydroxyapatite-forming coating for biomedical implants based on polypeptide-functionalized titania nanospikes

Titanium (Ti)-based implants often suffer from detrimental bacterial adhesion and inefficient healing, so it is crucial to design a dual-functional coating that prevents bacterial infection and enhances bioactivity for a successful implant. Herein, we successfully devised a cationic polypeptide (Pep)-functionalized biomimetic nanostructure coating with superior activity, which could not only kill pathogenic bacteria rapidly and inhibit biofilm formation for up to two weeks, but also promote in situ hydroxyapatite (HAp) formation. Specifically, a titania (TiO2) nanospike coating (TNC) was fabricated by alkaline hydrothermal treatment firstly, followed by immobilization of rationally synthesized Pep via robust coordinative interactions, named TNPC. This coating was able to effectively kill (>99.9%) both Gram-positive Staphylococcus aureus (S. aureus) and Gram-negative Escherichia coli (E. coli) bacteria, while being non-toxic to murine MC3T3-E1 osteoblastic cells. Furthermore, the in vivo infection studies denoted that the adherent bacteria numbers on the TNPC implants were significantly reduced by 6 orders of magnitude than those on the pure Ti implants (p < 0.001). Importantly, in the presence of cationic amino groups and residual Ti-OH groups, substantial HAp deposition on the TNPC surface in Kokubo's simulated body fluid (SBF) occurred after 14 days. Altogether, our results support the clinical potential of this biomimetic dual-functional coating as a new approach with desirable antibacterial properties and HAp-forming ability in orthopedic and dental applications.

Natural protein bioinspired materials for regeneration of hard tissues

This review describes the protein bioinspired materials for the repair of hard tissues such as enamel, dentin and bone.

Studying NAD(P)H cofactor-binding to alcohol dehydrogenases through global analysis of circular dichroism spectra

The initial step in reactions catalyzed by NAD(P)H-dependent alcohol dehydrogenases (ADHs) is the binding of the cofactor to the active site. To study this process, we measured NAD(P)H concentration-dependent circular dichroism (CD) in the presence of purified enzymes (ADH from horse liver, HLADH; ADH-A from Rhodococcus ruber; YGL157w from Saccharomyces cerevisiae) or enzyme-containing whole cell extract (ADH from Lactobacillus brevis, LbADH). We determined the proportions of binding and non-binding NAD(P)H and the associated dissociation constants (Kd) from matrix least-squares global fitting of law of mass action-derived model. Furthermore, the fitting allowed the back calculation of CD spectra corresponding to the cofactor in its bound conformation. With increasing pH and/or increasing ionic strength, we detected an increase in Kd for the NADH·HLADH complex with the shape of the bound cofactor conformation spectrum remaining unaffected. While the bound cofactor spectrum for the ADH-A·NADH complex was similar to that for HLADH, the corresponding spectra obtained for the NADPH-dependent enzymes YGL157w and LbADH exhibited opposite signs of the most prominent band. In comparison to CD spectra calculated on cofactor geometries from the crystal structures at the sTD-DFT level, we found that the sign of the bound cofactor spectrum correlates with the orientation of the nicotinamide ring of the cofactor in the active site. These results demonstrate the usefulness of the global analysis of cofactor titration CD spectra to study the role of cofactor binding and its geometry in ADH catalysis.

La-Doped biomimetic scaffolds facilitate bone remodelling by synchronizing osteointegration and phagocytic activity of macrophages

The capacity of osteoconduction held by HA/CS, osteoinduction by La³⁺, and biodegradability by a La-HA/CS composite, contributes to an ideal scaffold for osteointegration and remodelling.

Hyperstimulation of CaSR in human MSCs by biomimetic apatite inhibits endochondral ossification via temporal down-regulation of PTH1R

In adult bone injuries, periosteum-derived mesenchymal stem/stromal cells (MSCs) form bone via endochondral ossification (EO), whereas those from bone marrow (BM)/endosteum form bone primarily through intramembranous ossification (IMO). We hypothesized that this phenomenon is influenced by the proximity of MSCs residing in the BM to the trabecular bone microenvironment. Herein, we investigated the impact of the bone mineral phase on human BM-derived MSCs' choice of ossification pathway, using a biomimetic bone-like hydroxyapatite (BBHAp) interface. BBHAp induced hyperstimulation of extracellular calcium-sensing receptor (CaSR) and temporal down-regulation of parathyroid hormone 1 receptor (PTH1R), leading to inhibition of chondrogenic differentiation of MSCs even in the presence of chondroinductive factors, such as transforming growth factor-β1 (TGF-β1). Interestingly rescuing PTH1R expression using human PTH fragment (1-34) partially restored chondrogenesis in the BBHAp environment. In vivo studies in an ectopic site revealed that the BBHAp interface inhibits EO and strictly promotes IMO. Furthermore, CaSR knockdown (CaSR KD) disrupted the bone-forming potential of MSCs irrespective of the absence or presence of the BBHAp interface. Our findings confirm the expression of CaSR in human BM-derived MSCs and unravel a prominent role for the interplay between CaSR and PTH1R in regulating MSC fate and the choice of pathway for bone formation.

Secondary structure and dynamics study of the intrinsically disordered silica-mineralizing peptide P5 S3 during silicic acid condensation and silica decondensation

The silica forming repeat R5 of sil1 from Cylindrotheca fusiformis was the blueprint for the design of P5 S3 , a 50-residue peptide which can be produced in large amounts by recombinant bacterial expression. It contains 5 protein kinase A target sites and is highly cationic due to 10 lysine and 10 arginine residues. In the presence of supersaturated orthosilicic acid P5 S3 enhances silica-formation whereas it retards the dissolution of amorphous silica (SiO2 ) at globally undersaturated concentrations. The secondary structure of P5 S3 during these 2 processes was studied by circular dichroism (CD) spectroscopy, complemented by nuclear magnetic resonance (NMR) spectroscopy of the peptide in the absence of silicate. The NMR studies of dual-labeled (13 C, 15 N) P5 S3 revealed a disordered structure at pH 2.8 and 4.5. Within the pH range of 4.5-9.5 in the absence of silicic acid, the CD data showed a disordered structure with the suggestion of some polyproline II character. Upon silicic acid polymerization and during dissolution of preformed silica, the CD spectrum of P5 S3 indicated partial transition into an α-helical conformation which was transient during silica-dissolution. The secondary structural changes observed for P5 S3 correlate with the presence of oligomeric/polymeric silicic acid, presumably due to P5 S3 -silica interactions. These P5 S3 -silica interactions appear, at least in part, ionic in nature since negatively charged dodecylsulfate caused similar perturbations to the P5 S3 CD spectrum as observed with silica, while uncharged ß-d-dodecyl maltoside did not affect the CD spectrum of P5 S3 . Thus, with an associated increase in α-helical character, P5 S3 influences both the condensation of silicic acid into silica and its decondensation back to silicic acid.