Role of 20-kDa amelogenin (P148) phosphorylation in calcium phosphate formation in vitro

Biomimetic Randall's plaque as an in vitro model system for studying the role of acidic biopolymers in idiopathic stone formation

Randall's plaque (RP) deposits seem to be consistent among the most common type of kidney stone formers, idiopathic calcium oxalate stone formers. This group forms calcium oxalate renal stones without any systemic symptoms, which contributes to the difficulty of understanding and treating this painful and recurring disease. Thus, the development of an in vitro model system to study idiopathic nephrolithiasis, beginning with RP pathogenesis, can help in identifying how plaques and subsequently stones form. One main theory of RP formation is that calcium phosphate deposits initially form in the basement membrane of the thin loops of Henle, which then fuse and spread into the interstitial tissue, and ultimately make their way across the urothelium, where upon exposure to the urine, the mineralized tissue serves as a nidus for overgrowth with calcium oxalate into a stone. Our group has found that many of the unusual morphologies found in RP and stones, such as concentrically laminated spherulites and mineralized collagenous tissue, can be reproduced in vitro using a polymer-induced liquid precursor (PILP) process, in which acidic polypeptides induce a liquid phase amorphous precursor to the mineral, yielding non-equilibrium crystal morphologies. Given that there are many acidic proteins and polysaccharides present in the renal tissue and urine, we have put forth the hypothesis that the PILP system may be involved in urolithiasis. Therefore, our goal is to develop an in vitro model system of these two stages of composite stone formation to study the role that various acidic macromolecules may play. In our initial experiments presented here, the development of "biomimetic" RP was investigated, which will then serve as a nidus for calcium oxalate overgrowth studies. To mimic the tissue environment, MatriStem(®) (ACell, Inc.), a decellularized porcine urinary bladder matrix was used, because it has both an intact epithelial basement membrane surface and a tunica propria layer, thus providing the two types of matrix constituents found associated with mineral in the early stages of RP formation. We found that when using the PILP process to mineralize this tissue matrix, the two sides led to dramatically different mineral textures, and they bore a striking resemblance to native RP, which was not seen in the tissue mineralized via the classical crystal nucleation and growth process. The interstitium side predominantly consisted of collagen-associated mineral, while the luminal side had much less mineral, which appeared to be tiny spherules embedded within the basement membrane. Although these studies are only preliminary, they support our hypothesis that kidney stones may involve non-classical crystallization pathways induced by the large variety of macromolecular species in the urinary environment. We believe that mineralization of native tissue scaffolds is useful for developing a model system of stone formation, with the ultimate goal of developing strategies to avoid RP and its detrimental consequences in stone formation, or developing therapeutic treatments to prevent or cure the disease. Supported by NIDDK grant RO1DK092311.

Regulation of calcium phosphate formation by native amelogenins in vitro

Our previous in vitro studies have shown that recombinant full-length porcine amelogenin rP172 can transiently stabilize amorphous calcium phosphate (ACP) and uniquely guide the formation of well-aligned bundles of hydroxyapatite (HA) crystals, as seen in the secretory stage of amelogenesis. This functional capacity is dependent on the hydrophilic C-terminal domain of full-length amelogenin. However, we have also found that native phosphorylated (single S-16 site) forms of full-length (P173) and C-terminal cleaved (P148) amelogenins can stabilize ACP for > 2 d and prevent HA formation. The present study was carried out to test the hypothesis that, at reduced concentrations, native full-length P173 also has the capacity to guide ordered HA formation. The effect of P148 and P173 concentrations (0.2-2.0 mg/ml) on the rate of spontaneous calcium phosphate precipitation was monitored via changes in solution pH, while mineral phases formed were assessed using TEM. At higher P173 concentrations (1.0-2.0 mg/ml), limited mineral formation occurred and only ACP nanoparticles were observed during a 48 h period. However, at 0.4 mg/ml P173, a predominance of organized bundles of linear, needle-like HA crystals were observed. At 0.2 mg/ml of P173, limited quantities of less organized HA crystals were found. Although P148 similarly stabilized ACP, it did not guide ordered HA formation, like P173. Hence, the establishment of the hierarchical enamel structure during secretory stage amelogenesis may be regulated by the partial removal of full-length amelogenin via MMP20 proteolysis, while predominant amelogenin degradation products, like P148, serve to prevent uncontrolled mineral formation.

The flexible structure of the K24S28 region of Leucine-Rich Amelogenin Protein (LRAP) bound to apatites as a function of surface type, calcium, mutation, and ionic strength

Leucine-Rich Amelogenin Protein (LRAP) is a member of the amelogenin family of biomineralization proteins, proteins which play a critical role in enamel formation. Recent studies have revealed the structure and orientation of the N- and C-terminus of LRAP bound to hydroxyapatite (HAP), a surface used as an analog of enamel. The structure of one region, K24 to S28, was found to be sensitive to phosphorylation of S16, the only naturally observed site of serine phosphorylation in LRAP, suggesting that K24S28 may sit at a key region of structural flexibility and play a role in the protein's function. In this work, we investigated the sensitivity of the structure and orientation of this region when bound to HAP as a function of several factors which may vary during enamel formation to influence structure: the ionic strength (0.05, 0.15, 0.2 M), the calcium concentration (0.07 and 0.4 mM), and the surface to which it is binding [HAP and carbonated apatite (CAP), a more direct mimic of enamel]. A naturally occurring mutation found in amelogenin (T21I) was also investigated. The structure in the K24S28 region of the protein was found to be sensitive to these conditions, with the CAP surface and excess Ca(2+) (8:1 [Ca(2+)]:[LRAP-K24S28(+P)]) resulting in a tighter helix, while low ionic strength relaxed the helical structure. Higher ionic strength and the point mutation did not result in any structural change in this region. The distance of the backbone of K24 from the surface was most sensitive to excess Ca(2+) and in the T21I-mutation. Collectively, these data suggest that phosphorylated LRAP is able to accommodate structural changes while maintaining its interaction with the surface, and provides further evidence of the structural sensitivity of the K24S28 region, a sensitivity that may contribute to function in biomineralization.

CryoTEM study of effects of phosphorylation on the hierarchical assembly of porcine amelogenin and its regulation of mineralization in vitro

Amelogenin, the major extracellular enamel matrix protein, plays a critical role in regulating the growth and organization of enamel. Assembly and mineralization of full-length native (P173) and recombinant (rP172) porcine amelogenins were studied by cryogenic Transmission Electron Microscopy (cryoTEM). The cryoTEM revealed that both native and recombinant porcine amelogenins undergo step-wise self-assembly. Although the overall structural organization of P173 and rP172 oligomers was similar and resembled oligomers of murine recombinant amelogenin rM179, there were subtle differences suggesting that a single phosphorylated serine present in P173 might affect amelogenin self-assembly. Our mineralization studies demonstrated that both P173 and rP172 oligomers stabilize initial mineral clusters. Importantly, however, rP172 regulated the organization of initial mineral clusters into linear chains and guided the formation of parallel arrays of elongated mineral particles, which are the hallmark of enamel structural organization. These results are similar to those obtained previously using full-length recombinant murine amelogenin (Fang et al., 2011a). In contrast to that seen with rP172, phosphorylated P173 strongly inhibits mineralization for extended periods of time. We propose that these differences might be due to the differences in the structural organization and charge distribution between P173 and rP172. Overall our studies indicate that self-assembly of amelogenin and the mechanisms of its control over mineralization might be universal across different mammalian species. Our data also provide new insight into the effect of phosphorylation on amelogenin self-assembly and its regulation of mineralization.

Phosphorylation and ionic strength alter the LRAP-HAP interface in the N-terminus

The conditions present during enamel crystallite development change dramatically as a function of time, including the pH, protein concentration, surface type, and ionic strength. In this work, we investigate the role that two of these changing conditions, pH and ionic strength, have in modulating the interaction of the amelogenin, LRAP, with hydroxyapatite (HAP). Using solid-state NMR dipolar recoupling and chemical shift data, we investigate the structure, orientation, and dynamics of three regions in the N-terminus of the protein: L(15) to V(19), V(19) to L(23), and K(24) to S(28). These regions are also near the only phosphorylated residue in the protein pS(16); therefore, changes in the LRAP-HAP interaction as a function of phosphorylation (LRAP(-P) vs LRAP(+P)) were also investigated. All of the regions and conditions studied for the surface immobilized proteins showed restricted motion, with indications of slightly more mobility under all conditions for L(15)(+P) and K(24)(-P). The structure and orientation of the LRAP-HAP interaction in the N-terminus of the phosphorylated protein is very stable to changing solution conditions. From REDOR dipolar recoupling data, the structure and orientation in the region L(15)V(19)(+P) did not change significantly as a function of pH or ionic strength. The structure and orientation of the region V(19)L(23)(+P) were also stable to changes in pH, with the only significant change observed at high ionic strength, where the region becomes extended, suggesting this may be an important region in regulating mineral development. Chemical shift studies also suggest minimal changes in all three regions studied for both LRAP(-P) and LRAP(+P) as a function of pH or ionic strength, and also reveal that K(24) has multiple resolvable resonances, suggestive of two coexisting structures. Phosphorylation also alters the LRAP-HAP interface. All of the three residues investigated (L(15), V(19), and K(24)) are closer to the surface in LRAP(+P), but only K(24)S(28) changes structure as a result of phosphorylation, from a random coil to a largely helical structure, and V(19)L(23) becomes more extended at high ionic strength when phosphorylated. These observations suggest that ionic strength and dephosphorylation may provide switching mechanisms to trigger a change in the function of the N-terminus during enamel development.

C-terminal amidation of an osteocalcin-derived peptide promotes hydroxyapatite crystallization

Genesis of natural biocomposite-based materials, such as bone, cartilage, and teeth, involves interactions between organic and inorganic systems. Natural biopolymers, such as peptide motif sequences, can be used as a template to direct the nucleation and crystallization of hydroxyapatite (HA). In this study, a natural motif sequence consisting of 13 amino acids present in the first helix of osteocalcin was selected based on its calcium binding ability and used as substrate for nucleation of HA crystals. The acidic (acidic osteocalcin-derived peptide (OSC)) and amidic (amidic osteocalcin-derived peptide (OSN)) forms of this sequence were synthesized to investigate the effects of different C termini on the process of biomineralization. Electron microscopy analyses show the formation of plate-like HA crystals with random size and shape in the presence of OSN. In contrast, spherical amorphous calcium phosphate is formed in the presence of OSC. Circular dichroism experiments indicate conformational changes of amidic peptide to an open and regular structure as a consequence of interaction with calcium and phosphate. There is no conformational change detectable in OSC. It is concluded that HA crystal formation, which only occurred in OSN, is attributable to C-terminal amidation of a natural peptide derived from osteocalcin. It is also proposed that natural peptides with the ability to promote biomineralization have the potential to be utilized in hard tissue regeneration.

Secreted protein kinases

Protein kinases constitute one of the largest gene families and control many aspects of cellular life. In retrospect, the first indication for their existence was reported 130 years ago when the secreted protein, casein, was shown to contain phosphate. Despite its identification as the first phosphoprotein, the responsible kinase has remained obscure. This conundrum was solved with the discovery of a novel family of atypical protein kinases that are secreted and appear to phosphorylate numerous extracellular proteins, including casein. Fam20C, the archetypical member, phosphorylates secreted proteins within Ser-x-Glu/pSer motifs. This discovery has solved a 130-year-old mystery and has shed light on several human disorders of biomineralization.

A post-classical theory of enamel biomineralization… and why we need one

Enamel crystals are unique in shape, orientation and organization. They are hundreds of thousands times longer than they are wide, run parallel to each other, are oriented with respect to the ameloblast membrane at the mineralization front and are organized into rod or interrod enamel. The classical theory of amelogenesis postulates that extracellular matrix proteins shape crystallites by specifically inhibiting ion deposition on the crystal sides, orient them by binding multiple crystallites and establish higher levels of crystal organization. Elements of the classical theory are supported in principle by in vitro studies; however, the classical theory does not explain how enamel forms in vivo. In this review, we describe how amelogenesis is highly integrated with ameloblast cell activities and how the shape, orientation and organization of enamel mineral ribbons are established by a mineralization front apparatus along the secretory surface of the ameloblast cell membrane.

Structural changes in amelogenin upon self-assembly and mineral interactions

Amelogenin, the major protein of forming dental enamel, plays a crucial role in the biomineralization of this tissue. Amelogenin is soluble at low pH and self-assembles to form higher order structures at physiological pH. To understand the mechanisms of its assembly and interactions with calcium phosphate mineral, we conducted FTIR spectroscopy (FTIRS) studies of pH-triggered assembly of recombinant porcine amelogenin rP172 and its interactions with mature hydroxyapatite and apatitic mineral formed in situ. Analysis of our data indicated that rP172 at pH 3.0 exists in an unfolded disordered state, while increases in pH led to structural ordering, manifested by increases in intra- and intermolecular β-sheet structures and a decrease in random coil and β-turns. Amelogenin assembled at pH 7.2 was also found to contain large portions of extended intramolecular β-sheet and PPII. These FTIRS findings are consistent with those previously obtained with other techniques, thus verifying the validity of our experimental approach. Interestingly, interactions with mineral led to a reduction in protein structural organization. The findings obtained show that amelogenin has intrinsic structural flexibility to accommodate interactions with both forming and mature calcium phosphate mineral phases, providing new insights into the potential importance of amelogenin-mineral interactions in enamel biomineralization.

Quartz crystal microbalance analysis of DNA-templated calcium phosphate mineralization

A quartz crystal microbalance (QCM) sensor was developed for the quantitation of calcium phosphate mineralization and the assessment of DNA as a template molecule. Inherent advantages of QCM, such as nanogram sensitivity, temporal resolution, surface-based measurements, and flow capabilities, were leveraged in the design of this sensor, and in-line fluidic mixing was used to control precursor reaction. This research shows that DNA, a highly programmable anionic polymer, is able to template and control mineralization of calcium phosphate, with nucleation occurring in less than 15 min and initial rates ranging from 4 to 8 ng/min. FT-IR measurements show mineralized material to be calcium phosphate resembling hydroxyapatite (HAP) when a DNA template is used. DNA is a promising mineralization template, and the QCM proves to be a dynamic technique for a broad range of heterogeneous mineralization experiments in complement to classic, diffusion-limited, end-point analysis techniques.

Analogous effects of recombinant human full-length amelogenin expressed by Pichia pastoris yeast and enamel matrix derivative in vitro

OBJECTIVES

Amelogenins are proposed to be responsible for enamel matrix derivative (EMD)-induced periodontal regeneration; however, heterogeneity of amelogenins makes it challenging to purify the full-length proteins. This study has been carried out to express and purify a recombinant full-length human amelogenin protein (rHhAm175) in the eukaryotic yeast Pichia pastoris, and further compare biological responses of human periodontal ligament fibroblasts (PDLFs) to rHhAm175 and porcine EMD (pEMD).

MATERIALS AND METHODS

Human cDNA encoding a 175-amino acid amelogenin was subcloned into the pPIC3.5K vector. The rHhAm175 expressed in P. pastoris GS115 (Mut+) was purified and characterized. We examined cell attachment, migration and proliferation responses of human PDLFs to rHhAm175 and pEMD respectively, and characterized associated changes of proliferation-related intracellular signalling molecules, including extracellular signal response kinase (ERK) and Akt kinases/protein kinase B (Akt/PKB) kinases.

RESULTS

The purified rHhAm175 was confirmed to be molecular mass 22 021.13 Da, phosphorylated human amelogenin, and alone significantly promoted proliferation and migration of human PDLFs to an extent comparable to that of pEMD. Cell attachment was increased over the first 60 min incubation with rHhAm175 or pEMD. Both rHhAm175 and pEMD induced PDLF mitogenesis via extracellular signal response kinase (ERK1/2), but not by Akt kinases/protein kinase B (Akt/PKB).

CONCLUSIONS

rHhAm175 modulated cell activities of human PDLFs, to a comparable extent as porcine EMD. These data suggest that rHhAm175 might be used to induce periodontal tissue regeneration.

Effect of phosphorylation on the interaction of calcium with leucine-rich amelogenin peptide

Amelogenin undergoes self-assembly and plays an essential role in guiding enamel mineral formation. The leucine-rich amelogenin peptide (LRAP) is an alternative splice product of the amelogenin gene and is composed of the N terminus (containing the only phosphate group) and the C terminus of full-length amelogenin. This study was conducted to investigate further the role of phosphorylation in LRAP self-assembly in the presence and absence of calcium using small angle X-ray scattering (SAXS). Consistent with our previous dynamic light-scattering findings for phosphorylated (+P) and non-phosphorylated (-P) LRAP, SAXS analyses revealed radii of gyration (R(g)) for LRAP(-P) (46.3-48.0 Å) that were larger than those for LRAP(+P) (25.0-27.4 Å) at pH 7.4. However, added calcium (up to 2.5 mM) induced significant increases in the R(g) of LRAP(+P) (up to 46.4 Å), while it had relatively little effect on LRAP(-P) particle size. Furthermore, SAXS analyses suggested compact folded structures for LRAP(-P) in the presence and absence of calcium, whereas the conformation of LRAP(+P) changed from an unfolded structure to a more compact structure upon the addition of calcium. We conclude that the single phosphate group in LRAP(+P) induces functionally important conformational changes, suggesting that phosphorylation may also influence amelogenin conformation and protein-mineral interactions during the early stages of amelogenesis.

Self-assembly of amelogenin proteins at the water-oil interface

Self-assembly of amelogenin plays a key role in controlling enamel biomineralization. Recently, we generated self-aligning nanoribbons of amelogenin in water-in-oil emulsions stabilized by the full-length protein (rH174). Here, we tested the hypothesis that the hydrophilic C-terminus is critical for self-assembly of amelogenin into nanoribbons. The self-assembled structures of two amelogenin cleavage products, rH163 and rH146, were compared with structures of rH174 at different pH values and degrees of saturation using atomic force microscopy, electron microscopy, and dynamic light scattering. We observed that the number density of rH174 nanoribbons increased significantly when the initial pH was raised from 4.5 to 5.6. Nanoribbons, as well as unique helical nanostructures, were also readily observed when amelogenin rH146 was used, but showed little tendency for parallel alignment and did not bundle into fibrils like rH174. In contrast, rH163 rarely formed nanoribbons but predominantly assembled into nanospheres under the same conditions. We conclude that the presence of a hydrophilic C-terminus may not be a prerequisite for nanoribbon formation but may be critical for ribbon alignment and subsequent fibril formation. These results highlight the contribution of the hydrophobic domain in the self-assembly of elongated structures of amelogenins. Molecular mechanisms governing these processes based on the formation of reverse micelles are discussed.

Regulation of calcium phosphate formation by amelogenins under physiological conditions

Amelogenin is essential for proper enamel formation. The present in vitro study extends our previous work at low (10 mM) ionic strength (IS) by examining the effect of amelogenin on mineralization under higher (162 mM) IS conditions found in developing enamel. Full-length phosphorylated (P173) and non-phosphorylated (rP172) amelogenins were examined, along with P148 and rP147 that lack the hydrophilic C-terminus. Calcium phosphate formation was assessed by pH change, while the minerals formed were characterized using transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy. Amelogenin self-assembly was also studied using dynamic light scattering and TEM. The results indicate that IS does not influence the effects of rP147, rP172, and P173 on mineralization. However, in contrast to the findings for low IS, where both P173 and P148 stabilize initially formed amorphous calcium phosphate (ACP) nanoparticles for >1 d, elongated hydroxyapatite crystals were observed after 24 h using P148 at high IS, unlike that seen with P173. Differences in self-assembly help explain these findings, which suggest that P173 and P148 may play different roles in regulating enamel mineral formation. The present data support the notion that proteolytic processing of P173 is required in vivo to induce the transformation of initial ACP phases to apatitic enamel crystals.

Protein-mediated enamel mineralization

Enamel is a hard nanocomposite bioceramic with significant resilience that protects the mammalian tooth from external physical and chemical damages. The remarkable mechanical properties of enamel are associated with its hierarchical structural organization and its thorough connection with underlying dentin. This dynamic mineralizing system offers scientists a wealth of information that allows the study of basic principels of organic matrix-mediated biomineralization and can potentially be utilized in the fields of material science and engineering for development and design of biomimetic materials. This chapter will provide a brief overview of enamel hierarchical structure and properties and the process and stages of amelogenesis. Particular emphasis is given to current knowledge of extracellular matrix protein and proteinases, and the structural chemistry of the matrix components and their putative functions. The chapter will conclude by discussing the potential of enamel for regrowth.

Hierarchical self-assembly of amelogenin and the regulation of biomineralization at the nanoscale

Enamel is a highly organized hierarchical nanocomposite, which consists of parallel arrays of elongated apatitic crystallites forming an intricate three-dimensional microstructure. Amelogenin, the major extracellular matrix protein of dental enamel, regulates the formation of these crystalline arrays via cooperative interactions with forming mineral phase. Using cryoelectron microscopy, we demonstrate that amelogenin undergoes stepwise hierarchical self-assembly. Furthermore, our results indicate that interactions between amelogenin hydrophilic C-terminal telopeptides are essential for oligomer formation and for subsequent steps of hierarchical self-assembly. We further show that amelogenin assemblies stabilize mineral prenucleation clusters and guide their arrangement into linear chains that organize as parallel arrays. The prenucleation clusters subsequently fuse together to form needle-shaped mineral particles, leading to the formation of bundles of crystallites, the hallmark structural organization of the forming enamel at the nanoscale. These findings provide unique insight into the regulation of biological mineralization by specialized macromolecules and an inspiration for bottom-up strategies for the materials design.

Biominerals--hierarchical nanocomposites: the example of bone

Many organisms incorporate inorganic solids in their tissues to enhance their functional, primarily mechanical, properties. These mineralized tissues, also called biominerals, are unique organo-mineral nanocomposites, organized at several hierarchical levels, from nano- to macroscale. Unlike man-made composite materials, which often are simple physical blends of their components, the organic and inorganic phases in biominerals interface at the molecular level. Although these tissues are made of relatively weak components under ambient conditions, their hierarchical structural organization and intimate interactions between different elements lead to superior mechanical properties. Understanding basic principles of formation, structure, and functional properties of these tissues might lead to novel bioinspired strategies for material design and better treatments for diseases of the mineralized tissues. This review focuses on general principles of structural organization, formation, and functional properties of biominerals on the example the bone tissues.

Leucine-rich amelogenin peptides regulate mineralization in vitro

Amelogenin's capacity to regulate enamel formation is related to its conserved N- and C-terminal domains, its ability to self-assemble, and its ability to stabilize amorphous calcium phosphate (ACP) - a capacity enhanced by amelogenin phosphorylation. This in vitro study provides further insight into amelogenin function, using variations of the Leucine-Rich Amelogenin Peptide (LRAP), an alternative splice product comprised solely of amelogenin's N- and C-terminal domains. Peptide self-assembly was studied by dynamic light-scattering and transmission electron microscopy (TEM). TEM, selected area electron diffraction, and Fourier transform-infrared spectroscopy were also used to determine the effect of phosphorylated and non-phosphorylated LRAP on calcium phosphate formation. Results show that phosphorylated and non-phosphorylated LRAP can self-assemble into chain-like structures in a fashion dependent on the C-terminal domain. Notably, this capacity was enhanced by added calcium and to a much greater degree for phosphorylated LRAP. Furthermore, phosphorylated LRAP was found to stabilize ACP and prevent its transformation to hydroxyapatite (HA), while aligned HA crystals formed in the presence of non-phosphorylated LRAP. The N- and C-terminal amelogenin domains in non-phosphorylated LRAP are, therefore, sufficient to guide ACP transformation into ordered bundles of apatite crystals, making LRAP an excellent candidate for biomimetic approaches for enamel regeneration.

Cryogenic transmission electron microscopy study of amelogenin self-assembly at different pH

Cryogenic transmission electron microscopy (cryo-EM) was used to explore the self-assembly of recombinant murine amelogenin (rM179) in vitro. Our cryo-EM data showed that amelogenin self-assembly is a strongly pH-dependent process. At pH 4.4 the main fraction of the protein exists in a monomeric form, although some peculiar structures consisting of chains of monomers were also observed. At pH 5.8 large nanospheres comprising ring-like structures ~50 nm in diameter were the most abundant particle class. Similarly, at pH 8.0 amelogenins self-assembled into ring-like oligomers of different sizes, which subsequently assembled into nanospheres 15-20 nm in diameter. Furthermore, at pH 7.2, which is close to a physiological pH, branched chains of nanospheres were observed. Our results show that amelogenin assembly is a multistep hierarchical process and provides new insight into the control of enamel mineralization.

Potential role of the amelogenin N-terminus in the regulation of calcium phosphate formation in vitro

N-terminal and C-terminal (CT) domains of amelogenin have been shown to be essential for proper enamel formation. Recent studies have also suggested that although the C-terminus plays an apparent role in protein-mineral interactions, other amelogenin structural domains are involved. The objective was to explore the role of the amelogenin N-terminus in the regulation of calcium phosphate formation in vitro. Spontaneous mineralization studies were carried out using the phosphorylated (+P) and nonphosphorylated (-P) N-terminus of the leucine-rich amelogenin peptide (LRAP) that lacks the hydrophilic CT domain. Mineralization progress was monitored via changes in solution pH. Mineral phases formed were characterized using TEM, selected area electron diffraction, and FT-IR. In controls, amorphous calcium phosphate was initially formed and subsequently transformed to randomly oriented hydroxyapatite (HA) plate-like crystals. In contrast to the control, LRAP(+P)-CT stabilized ACP formation for >1 day, while LRAP(-P)-CT accelerated the transformation of ACP to HA but had little effect on crystal shape or orientation. In conclusion, the N-terminal domain found in LRAP, as in amelogenins, appears to have the capacity to interact with forming calcium phosphate mineral phases. Results suggest that the N-terminal domain of amelogenin may play a direct role in early stages of enamel formation.

The cooperation of enamelin and amelogenin in controlling octacalcium phosphate crystal morphology

Enamel matrix proteins, including the most abundant amelogenin and lesser amounts of enamelin, ameloblastin, and proteinases, play vital roles in controlling crystal nucleation and growth during enamel formation. The cooperative action between amelogenin and the 32-kDa enamelin is critical to regulating the growth morphology of octacalcium phosphate crystals. Using biophysical methods, we investigated the interaction between the 32-kDa enamelin and recombinant pig amelogenin 148 (rP148) at pH 6.5 in phosphate-buffered saline (PBS). Dynamic light scattering results showed a trend of increasing particle size in the mixture with the addition of enamelin to amelogenin. Upon addition of the 32-kDa enamelin, the shift and intensity decrease in the ellipticity minima of rP148 in the circular dichroism spectra of rP148 illustrated a direct interaction between the 2 proteins. In the fluorescence spectra, the maximum emission of rP148 was blue shifted from 335 to 333 nm in the presence of enamelin as a result of complexation of the 2 proteins. Our results demonstrate that the 32-kDa enamelin has a close association with amelogenin at pH 6.5 in PBS buffer. Our present study provides novel insights into the possible cooperation between enamelin and amelogenin in macromolecular coassembly and in controlling enamel mineral formation.

Porcine Amelogenin : Alternative Splicing, Proteolytic Processing, Protein - Protein Interactions, and Possible Functions

Amelogenin is the major secretory product of ameloblasts and is critical for proper tooth enamel formation. Amelogenin isoforms and their cleavage products comprise over 80% of total secretory stage enamel protein. We have isolated and characterized four secreted amelogenin isoforms from developing porcine enamel : P190 (27-kDa), P173 (25-kDa), P132 (18-kDa) and P56 (6.5-kDa ; leucine rich amelogenin polypeptide or LRAP). P190 and P132 are low abundance amelogenins that contain a novel exon 4-encoded segment of lack the exon 3-encoded segment, respectively. P173 is the most abundant (major) amelogenin isoform. Cleavage of P173 by matrix metalloproteinase 20 (Mmp20) occurs at specific sites that generates a set of N-terminal cleavage products : P162 (23-kDa), P148 (20-kDa), P62/P63 (11-kDa), and Trp(45) (6-kDa, tyrosine rich amelogenin polypeptide or TRAP). P148 is the most abundant protein in developing enamel and influences the conversion of amorphous calcium phosphate into hydroxyapatite in vitro. Mmp20 cleaves LRAP, the second abundant amelogenin isoform after Pro(45) and Pro(40). Processing by Mmp20 allows amelogenin cleavage products to serve separate functions. Over time, Mmp20 catalyzes additional cleavages that facilitate the progressive replacement of amelogenin by mineral, so enamel crystals thicken and widen with depth. Besides proteolytic processing, amelogenin protein-protein interactions are critical for function. Far-Western analyses demonstrate that the larger amelogenins (P173, P162, and P148) are only able to interact with larger amelogenins. No amelogenin-amelogenin interactions are observed for the smaller amelogenin cleavage products, TRAP or LRAP Amelogenin doesn't interact with the 32-kDa glycosylated enamelin cleavage product, unless it it partially deglycosylated.

MMS6 protein regulates crystal morphology during nano-sized magnetite biomineralization in vivo

Biomineralization, the process by which minerals are deposited by organisms, has attracted considerable attention because this mechanism has shown great potential to inspire bottom-up material syntheses. To understand the mechanism for morphological regulation that occurs during biomineralization, many regulatory proteins have been isolated from various biominerals. However, the molecular mechanisms that regulate the morphology of biominerals remain unclear because there is a lack of in vivo evidence. Magnetotactic bacteria synthesize intracellular magnetosomes that comprise membrane-enveloped single crystalline magnetite (Fe(3)O(4)). These nano-sized magnetite crystals (<100 nm) are bacterial species dependent in shape and size. Mms6 is a protein that is tightly associated with magnetite crystals. Based on in vitro experiments, this protein was first implicated in morphological regulation during nano-sized magnetite biomineralization. In this study, we analyzed the mms6 gene deletion mutant (Δmms6) of Magnetospirillum magneticum (M. magneticum) AMB-1. Surprisingly, the Δmms6 strain was found to synthesize the smaller magnetite crystals with uncommon crystal faces, while the wild-type and complementation strains synthesized highly ordered cubo-octahedral crystals. Furthermore, deletion of mms6 gene led to drastic changes in the profiles of the proteins tightly bound to magnetite crystals. It was found that Mms6 plays a role in the in vivo regulation of the crystal structure to impart the cubo-octahedral morphology to the crystals during biomineralization in magnetotactic bacteria. Magnetotactic bacteria synthesize magnetite crystals under ambient conditions via a highly controlled morphological regulation system that uses biological molecules.

Self-aligning amelogenin nanoribbons in oil-water system

The highly organized microstructure of dental enamel is a result of protein-guided anisotropic growth of apatite nanofibers. It is established that amelogenin proteins, the main constituent of the developing enamel matrix, form nanospheres in vitro, but the amphiphilic nature of the full-length protein conveys the possibility of generating more complex structures as observed with other surfactant-like molecules. This study tested if the use of metastable oil-water emulsions can induce supramolecular assemblies of amelogenin. Recombinant full-length amelogenin, rH174, was mixed into octanol/ethyl acetate preparations of different ratios to form emulsions at pH 4.5 and 7.4. Atomic force and electron microscopy showed the formation of 16.7±1.0nm wide nanoribbons which grew to several micrometer length over a period of days. Nanoribbons formed from reverse micelles by enabling hydrophobic tails of the molecules to interact while preventing the formation of amelogenin nanospheres. Ribbon formation required the presence of calcium and phosphate ions and may be localized at a dark central line along the amelogenin ribbons. The ribbons have a strong tendency to align in parallel maintaining 5-20nm space between each other. The growth rates and number of ribbons were significantly higher at pH 4.5 and related to the metastability of the emulsion. A model for ribbon extension proposes the addition of short segments or amelogenin dimers to the ends of the ribbon. The formation of self-aligning and uniaxially elongating amelogenin structures triggered by the presence of calcium and phosphate may represent a suitable new model for protein controlled mineralization in enamel.

Effects of phosphorylation on the self-assembly of native full-length porcine amelogenin and its regulation of calcium phosphate formation in vitro

The self-assembly of the predominant extracellular enamel matrix protein amelogenin plays an essential role in regulating the growth and organization of enamel mineral during early stages of dental enamel formation. The present study describes the effect of the phosphorylation of a single site on the full-length native porcine amelogenin P173 on self-assembly and on the regulation of spontaneous calcium phosphate formation in vitro. Studies were also conducted using recombinant non-phosphorylated (rP172) porcine amelogenin, along with the most abundant amelogenin cleavage product (P148) and its recombinant form (rP147). Amelogenin self-assembly was assessed using dynamic light scattering (DLS) and transmission electron microscopy (TEM). Using these approaches, we have shown that self-assembly of each amelogenin is very sensitive to pH and appears to be affected by both hydrophilic and hydrophobic interactions. Furthermore, our results suggest that the phosphorylation of the full-length porcine amelogenin P173 has a small but potentially important effect on its higher-order self-assembly into chain-like structures under physiological conditions of pH, temperature, and ionic strength. Although phosphorylation has a subtle effect on the higher-order assembly of full-length amelogenin, native phosphorylated P173 was found to stabilize amorphous calcium phosphate for extended periods of time, in sharp contrast to previous findings using non-phosphorylated rP172. The biological relevance of these findings is discussed.

Tooth enamel proteins enamelin and amelogenin cooperate to regulate the growth morphology of octacalcium phosphate crystals

To examine the hypothetical cooperative role of enamelin and amelogenin in controlling the growth morphology of enamel crystals in the post-secretory stage, we applied a cation selective membrane system for the growth of octacalcium phosphate (OCP) in the truncated recombinant porcine amelogenin (rP148) with and without the 32kDa enamelin fragment. Enamelin alone inhibited the growth in the c-axis direction more than rP148, yielding OCP crystals with the smallest aspect ratio of all conditions tested. When enamelin was added to the amelogenin "gel-like matrix", the inhibitory action of the protein mixture on the growth of OCP in the c-axis direction was diminished, while that in the b-axis direction was increased. As a result, the length to width ratio (aspect ratio) of OCP crystal was markedly increased. Addition of enamelin to amelogenin enhanced the potential of amelogenin to stabilize the amorphous calcium phosphate (ACP) transient phase. The ratio of enamelin and amelogenin was crucial for stabilization of ACP and the growth of OCP crystals with larger aspect ratio. The cooperative regulatory action of enamelin and amelogenin was attained, presumably, through co-assembling of enamelin and amelogenin. These results have important implications in understanding the growth mechanism of enamel crystals with large aspect ratio.

Hierarchical and non-hierarchical mineralisation of collagen

Biomineralisation of collagen involves functional motifs incorporated in extracellular matrix protein molecules to accomplish the objectives of stabilising amorphous calcium phosphate into nanoprecursors and directing the nucleation and growth of apatite within collagen fibrils. Here we report the use of small inorganic polyphosphate molecules to template hierarchical intrafibrillar apatite assembly in reconstituted collagen in the presence of polyacrylic acid to sequester calcium and phosphate into transient amorphous nanophases. The use of polyphosphate without a sequestration analogue resulted only in randomly-oriented extrafibrillar precipitations along the fibrillar surface. Conversely, the use of polyacrylic acid without a templating analogue resulted only in non-hierarchical intrafibrillar mineralisation with continuous apatite strands instead of discrete crystallites. The ability of using simple non-protein molecules to recapitulate different levels of structural hierarchy in mineralised collagen signifies the ultimate simplicity in Nature's biomineralisation design principles and challenges the need for using more complex recombinant matrix proteins in bioengineering applications.

Regulation of dental enamel shape and hardness

Epithelial-mesenchymal interactions guide tooth development through its early stages and establish the morphology of the dentin surface upon which enamel will be deposited. Starting with the onset of amelogenesis beneath the future cusp tips, the shape of the enamel layer covering the crown is determined by five growth parameters: the (1) appositional growth rate, (2) duration of appositional growth (at the cusp tip), (3) ameloblast extension rate, (4) duration of ameloblast extension, and (5) spreading rate of appositional termination. Appositional growth occurs at a mineralization front along the ameloblast distal membrane in which amorphous calcium phosphate (ACP) ribbons form and lengthen. The ACP ribbons convert into hydroxyapatite crystallites as the ribbons elongate. Appositional growth involves a secretory cycle that is reflected in a series of incremental lines. A potentially important function of enamel proteins is to ensure alignment of successive mineral increments on the tips of enamel ribbons deposited in the previous cycle, causing the crystallites to lengthen with each cycle. Enamel hardens in a maturation process that involves mineral deposition onto the sides of existing crystallites until they interlock with adjacent crystallites. Neutralization of acidity generated by hydroxyapatite formation is a key part of the mechanism. Here we review the growth parameters that determine the shape of the enamel crown as well as the mechanisms of enamel appositional growth and maturation.

Mineralogical signatures of stone formation mechanisms

The mechanisms involved in biomineralization are modulated through interactions with organic matrix. In the case of stone formation, the role of the organic macromolecules in the complex urinary environment is not clear, but the presence of mineralogical 'signatures' suggests that some aspects of stone formation may result from a non-classical crystallization process that is induced by acidic proteins. An amorphous precursor has been detected in many biologically controlled mineralization reactions, which is thought to be regulated by non-specific interactions between soluble acidic proteins and mineral ions. Using in vitro model systems, we find that a liquid-phase amorphous mineral precursor induced by acidic polypeptides can lead to crystal textures that resemble those found in Randall's plaque and kidney stones. This polymer-induced liquid-precursor process leads to agglomerates of coalesced mineral spherules, dense-packed spherulites with concentric laminations, mineral coatings and 'cements', and collagen-associated mineralization. Through the use of in vitro model systems, the mechanisms involved in the formation of these crystallographic features may be resolved, enhancing our understanding of the potential role(s) that proteins play in stone formation.

Amelogenin-collagen interactions regulate calcium phosphate mineralization in vitro

Collagen and amelogenin are two major extracellular organic matrix proteins of dentin and enamel, the mineralized tissues comprising a tooth crown. They both are present at the dentin-enamel boundary (DEB), a remarkably robust interface holding dentin and enamel together. It is believed that interactions of dentin and enamel protein assemblies regulate growth and structural organization of mineral crystals at the DEB, leading to a continuum at the molecular level between dentin and enamel organic and mineral phases. To gain insight into the mechanisms of the DEB formation and structural basis of its mechanical resiliency we have studied the interactions between collagen fibrils, amelogenin assemblies, and forming mineral in vitro, using electron microscopy. Our data indicate that collagen fibrils guide assembly of amelogenin into elongated chain or filament-like structures oriented along the long axes of the fibrils. We also show that the interactions between collagen fibrils and amelogenin-calcium phosphate mineral complexes lead to oriented deposition of elongated amorphous mineral particles along the fibril axes, triggering mineralization of the bulk of collagen fibril. The resulting structure was similar to the mineralized collagen fibrils found at the DEB, with arrays of smaller well organized crystals inside the collagen fibrils and bundles of larger crystals on the outside of the fibrils. These data suggest that interactions between collagen and amelogenin might play an important role in the formation of the DEB providing structural continuity between dentin and enamel.

Mapping amorphous calcium phosphate transformation into crystalline mineral from the cell to the bone in zebrafish fin rays

The continuously forming fin bony rays of zebrafish represent a simple bone model system in which mineralization is temporally and spatially resolved. The mineralized collagen fibrils of the fin bones are identical in structure to those found in all known bone materials. We study the continuous mineralization process within the tissue by using synchrotron microbeam x-ray diffraction and small-angle scattering, combined with cryo-scanning electron microscopy. The former provides information on the mineral phase and the mineral particles size and shape, whereas the latter allows high-resolution imaging of native hydrated tissues. The integration of the two techniques demonstrates that new mineral is delivered and deposited as packages of amorphous calcium phosphate nanospheres, which transform into platelets of crystalline apatite within the collagen matrix.

How amelogenin orchestrates the organization of hierarchical elongated microstructures of apatite

Amelogenin (Amel) accelerates the nucleation of hydroxyapatite (HAP) in supersaturated solutions of calcium phosphate (Ca-P), shortening the induction time (delay period), under near-physiological conditions of pH, temperature, and ionic strength. Hierarchically organized Amel and amorphous calcium phosphate (ACP) nanorod microstructures are formed involving a coassembly of Amel-ACP particles at low supersaturations and low protein concentrations in a slow, well-controlled, constant composition (CC) crystallization system. At the earliest nucleation stages, the CC method allows the capture of prenucleation clusters and intermediate nanoclusers, spherical nanoparticles, and nanochains prior to enamel-like nanorod microstructure formations at later maturation stages. Amel-ACP nanoscaled building blocks are formed spontaneously by synergistic interactions between flexible Amel protein molecules and Ca-P prenucleation clusters, and these spherical nanoparticles evolve by orientated aggregation to form nanochains. Our results suggest that, in vivo, Amel may determine the structure of enamel by controlling prenucleation cluster aggregation at the earliest stages by forming stable Amel-ACP microstructures prior to subsequent crystal growth and mineral maturation.