Enamel matrix protein turnover during amelogenesis: basic biochemical properties of short-lived sulfated enamel proteins

Protein Interaction between Ameloblastin and Proteasome Subunit α Type 3 Can Facilitate Redistribution of Ameloblastin Domains within Forming Enamel

Enamel is a bioceramic tissue composed of thousands of hydroxyapatite crystallites aligned in parallel within boundaries fabricated by a single ameloblast cell. Enamel is the hardest tissue in the vertebrate body; however, it starts development as a self-organizing assembly of matrix proteins that control crystallite habit. Here, we examine ameloblastin, a protein that is initially distributed uniformly across the cell boundary but redistributes to the lateral margins of the extracellular matrix following secretion thus producing cell-defined boundaries within the matrix and the mineral phase. The yeast two-hybrid assay identified that proteasome subunit α type 3 (Psma3) interacts with ameloblastin. Confocal microscopy confirmed Psma3 co-distribution with ameloblastin at the ameloblast secretory end piece. Co-immunoprecipitation assay of mouse ameloblast cell lysates with either ameloblastin or Psma3 antibody identified each reciprocal protein partner. Protein engineering demonstrated that only the ameloblastin C terminus interacts with Psma3. We show that 20S proteasome digestion of ameloblastin in vitro generates an N-terminal cleavage fragment consistent with the in vivo pattern of ameloblastin distribution. These findings suggest a novel pathway participating in control of protein distribution within the extracellular space that serves to regulate the protein-mineral interactions essential to biomineralization.

Biomineralization of a self-assembled-, soft-matrix precursor: Enamel

Enamel is the bioceramic covering of teeth, a composite tissue composed of hierarchical organized hydroxyapatite crystallites fabricated by cells under physiologic pH and temperature. Enamel material properties resist wear and fracture to serve a lifetime of chewing. Understanding the cellular and molecular mechanisms for enamel formation may allow a biology-inspired approach to material fabrication based on self-assembling proteins that control form and function. Genetic understanding of human diseases expose insight from Nature's errors by exposing critical fabrication events that can be validated experimentally and duplicated in mice using genetic engineering to phenocopy the human disease so that it can be explored in detail. This approach led to assessment of amelogenin protein self-assembly which, when altered, disrupts fabrication of the soft enamel protein matrix. A misassembled protein matrix precursor results in loss of cell to matrix contacts essential to fabrication and mineralization.

Mutations in the beta propeller WDR72 cause autosomal-recessive hypomaturation amelogenesis imperfecta

Healthy dental enamel is the hardest and most highly mineralized human tissue. Though acellular, nonvital, and without capacity for turnover or repair, it can nevertheless last a lifetime. Amelogenesis imperfecta (AI) is a collective term for failure of normal enamel development, covering diverse clinical phenotypes that typically show Mendelian inheritance patterns. One subset, known as hypomaturation AI, is characterised by near-normal volumes of organic enamel matrix but with weak, creamy-brown opaque enamel that fails prematurely after tooth eruption. Mutations in genes critical to enamel matrix formation have been documented, but current understanding of other key events in enamel biomineralization is limited. We investigated autosomal-recessive hypomaturation AI in a consanguineous Pakistani family. A whole-genome SNP autozygosity screen identified a locus on chromosome 15q21.3. Sequencing candidate genes revealed a point mutation in the poorly characterized WDR72 gene. Screening of WDR72 in a panel of nine additional hypomaturation AI families revealed the same mutation in a second, apparently unrelated, Pakistani family and two further nonsense mutations in Omani families. Immunohistochemistry confirmed intracellular localization in maturation-stage ameloblasts. WDR72 function is unknown, but as a putative β propeller is expected to be a scaffold for protein-protein interactions. The nearest homolog, WDR7, is involved in vesicle mobilization and Ca²⁺-dependent exocytosis at synapses. Vesicle trafficking is important in maturation-stage ameloblasts with respect to secretion into immature enamel and removal of cleaved enamel matrix proteins via endocytosis. This raises the intriguing possibility that WDR72 is critical to ameloblast vesicle turnover during enamel maturation.

Ultrastructural analyses of deciduous teeth affected by hypocalcified amelogenesis imperfecta from a family with a novel Y458X FAM83H nonsense mutation

BACKGROUND

Nonsense mutations in FAM83H are a recently described underlying cause of autosomal dominant (AD) hypocalcified amelogenesis imperfecta (AI).

OBJECTIVE

This study aims to report a novel c.1374C>A p.Y458X nonsense mutation and describe the associated ultrastructural phenotype of deciduous teeth.

METHODS

A family of European origin from the Iberian Peninsula with AD-inherited AI was ascertained. Family members were assessed through clinical examination and supporting investigations. Naturally exfoliated deciduous teeth from 2 siblings were investigated by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and transverse microradiography (TMR).

RESULTS

On clinical and radiographic investigation the appearances of the affected deciduous and permanent teeth were consistent with hypocalcified AI with small focal areas of more normal looking enamel. DNA sequencing identified a novel c.1374C>A p.Y458X FAM83H nonsense mutation in affected, but not in either unaffected family members or unrelated controls. Exfoliated teeth were characterised by substantial post-eruptive enamel loss on gross examination. Irregular, poor quality enamel prisms were observed on SEM. These were coated in amorphous material. TMR and EDX confirmed reduced mineral and increased organic content in enamel, respectively.

CONCLUSIONS

FAM83H nonsense mutations have recently been recognised as a cause of AD hypocalcified AI. We report a novel nonsense FAM83H mutation and describe the associated preliminary ultrastructural phenotype in deciduous teeth. This is characterised by poorly formed enamel rods with inappropriate retention of amorphous material, which is likely to represent retained organic matrix that contributes to the overall hypomineralised phenotype.

Different enamel and dentin mineralization observed in VDR deficient mouse model

Vitamin D plays an important role in the bone mineralization process. Enamel and dentin are two mineralized tissues of different origins that combine to form teeth, but the mechanism by which vitamin D regulates these tissues remains unclear. We hypothesized that vitamin D affects enamel and dentin mineralization through different mechanisms.

OBJECTIVE

To examine enamel and dentin mineralization in a vitamin D receptor (VDR) deficient mouse model by micro-computerized tomography (micro-CT) and scanning electronic microscopy (SEM).

METHODS

VDR wild type mice (VDR+/+) and VDR deficient (VDR-/-) littermates were sacrificed at 70.5 days old, and their mandibles were dissected. Micro-CT was used to compare mineral density (MD) of enamel and dentin of the two groups at different levels along the axis of mandibular incisors. SEM was employed to examine the ultrastructure of incisors at the levels corresponding to the levels used for the micro-CT studies. Furthermore, an accelerated eruption procedure was performed to exclude the effect of delayed eruption on enamel and dentin mineralization.

RESULTS

Different distribution patterns of enamel and dentin MD were observed between VDR+/+ and VDR-/- groups. Early enamel maturation, mineralization, and hypomineralization in dentin were observed in the VDR deficient mice.

CONCLUSION

Vitamin D may affect the mineralization of dentin systemically, and enamel mineralization may be regulated locally.

Enamel matrix proteins bind to wound matrix proteins and regulate their cell-adhesive properties

Enamel matrix proteins (EMP) induce periodontal regeneration and accelerate dermal wound healing, but the cellular mechanisms of these processes are unclear. We investigated the binding of EMP to the wound matrix proteins fibronectin, laminin-1, collagen type I, and collagen type IV and analyzed the interaction of epithelial cells and periodontal ligament fibroblasts (PDLF) with EMP and composite matrices of EMP + fibronectin or EMP + collagen. The adhesion of PDLF to EMP was concentration- and integrin-dependent and did not require de novo protein synthesis. EMP supported PDLF migration. In contrast, keratinocytes did not adhere to EMP if their protein synthesis was blocked. EMP showed concentration-dependent binding of fibronectin, peaking at 100 microg ml(-1) (before the precipitation point) of EMP. Type I collagen binding to EMP peaked at a low (1 microg ml(-1)) and narrow concentration range. Neither laminin-1 nor type IV collagen bound to EMP. Collagen and fibronectin, bound to EMP, showed significantly reduced (> 50%) binding of both epithelial cells and PDLF compared with the equivalent concentration of these proteins alone. PDLF, but not epithelial cell, adhesion was rescued by increasing the EMP concentration. These findings show that EMP binds to wound extracellular matrix proteins and regulates their adhesive properties. Such interactions may favor fibroblast adhesion over epithelial cells, potentially promoting connective tissue regeneration.

Protein–Protein Interactions of the Developing Enamel Matrix

Extracellular matrix proteins control the formation of the inorganic component of hard tissues including bone, dentin, and enamel. The structural proteins expressed primarily in the enamel matrix are amelogenin, ameloblastin, enamelin, and amelotin. Other proteins, like biglycan, are also present in the enamel matrix as well as in other mineralizing and nonmineralizing tissues of mammals. In addition, the presence of sulfated enamel proteins, and "tuft" proteins has been examined and discussed in relation to enamel formation. The structural proteins of the enamel matrix must have specific protein-protein interactions to produce a matrix capable of directing the highly ordered structure of the enamel crystallites. Protein-protein interactions are also likely to occur between the secreted enamel proteins and the plasma membrane of the enamel producing cells, the ameloblasts. Such protein-protein interactions are hypothesized to influence the secretion of enamel proteins, establish short-term order of the forming matrix, and to mediate feedback signals to the transcriptional machinery of these cells. Membrane-bound proteins identified in ameloblasts, and which interact with the structural enamel proteins, include Cd63 (cluster of differentiation 63 antigen), annexin A2 (Anxa2), and lysosomal-associated glycoprotein 1 (Lamp1). These and related data help explain the molecular and cellular mechanisms responsible for the removal of the organic enamel matrix during the events of enamel mineralization, and how the enamel matrix influences its own fate through signaling initiated at the cell surface. The knowledge gained from enamel developmental studies may lead to better dental and nondental materials, or materials inspired by Nature. These data will be critical to scientists, engineers, and dentists in their pursuits to regenerate an entire tooth. For tooth regeneration to become a reality, the protein-protein interactions involving the key dental proteins must be identified and understood. The scope of this review is to discuss the current understanding of protein-protein interactions of the developing enamel matrix, and relate this knowledge to enamel biomineralization.

Ameloblastin and amelogenin expression in posnatal developing mouse molars

Ameloblastin and amelogenin are structural proteins present in the enamel matrix of developing teeth. Here we report the results of in situ hybridization analyses with DNA probes of ameloblastin and amelogenin expression in the mandibular first molars of ICR/Jcl mice from postnatal day 1 to day 15. Ameloblastin mRNA expression was observed in ameloblasts at day 2 while amelogenin mRNA was detected in secretory ameloblasts at day 3. Significant expression of both molecules was observed at days 4, 5 and 6, after which the levels decreased. Amelogenin expression ended on day 10, while ameloblastin mRNA was only weakly detected on day 12. Neither amelogenin nor ameloblastin expression was observed in day 15 mouse molars. Amelogenin and ameloblastin mRNAs were restricted to ameloblasts. We conclude that amelogenin and ameloblastin expression is enamel-specific, and suggest that these genes might be involved in the mineralization of enamel. It is possible that ameloblastin could participate in the attachment of ameloblasts to the enamel surface. In this case, the downregulation of expression may indicate the beginning of the maturation stage in which the ameloblasts tend to detach from the enamel layer.

Biglycan is a repressor of amelogenin expression and enamel formation: an emerging hypothesis

The authors suggest that biglycan acts as a repressor of the expression of amelogenin in the two unique groups of cells involved in amelogenin synthesis, namely, the secretory ameloblasts and odontoblasts.

Calcium binding of enamel proteins and their derivatives with emphasis on the calcium-binding domain of porcine sheathlin

Dental enamel is believed to form by the transfer of ions from solution, primarily calcium, phosphate, hydroxyl and carbonate, to the surface of solid-state mineral. Such precipitation phenomena can be controlled by regulating the degree of saturation of the solution with respect to the potential solid phases that can form. The concentration of free calcium is the factor that most affects the degree of saturation for calcium hydroxyapatite, and its buffering by calcium-binding proteins has been proposed as the mechanism that determines the enamel mineral structure. In this study, Stains-all staining was used to identify and isolate calcium-binding proteins from the enamel matrix, and determine their structures and association constants for calcium. Proteolytic cleavage fragments derived from the C-terminus of sheathlin, having apparent molecular weights of 13, 15, 27 and 29 kDa, were characterized by amino-terminal protein sequencing, amino acid analysis, and sugar, phosphate and sulphate determinations. Sheathlin C-terminal cleavage products were shown to have no N-linked glycosylations or phosphorylated amino acids, but Pro(350) was hydroxylated, and there was one sulphated O-linked glycosylation at Thr(386), containing galactose and N-acetylgalactosamine. The calcium-binding association constants for enamel proteins ranged from a high of 1.2 x 10(4) M(-1) to a low of 4.4x10(1) M(-1). The relative strengths of binding in order of decreasing affinity were: 13 and 15 kDa calcium-binding domain of sheathlin >27 and 29 kDa calcium-binding proteins >32 kDa enamelin >89 kDa enamelin >6.5 kDa, 25 kDa, 23 kDa, 20 kDa, 13 kDa, 5.3 kDa amelogenins. It is concluded that if enamel proteins have similar calcium-binding properties in vivo as have been measured in vitro, they would tend to buffer the free calcium ion concentration in enamel fluid.

Is there more to enamel matrix proteins than biomineralization?

Enamel proteins are proteins synthesized by ameloblast cells. These proteins are secreted into the enamel extracellular matrix where they nucleate and regulate the growth of hydroxyapatite crystals to form the mineralized enamel covering the crown of the teeth. Although the exact role of these proteins in enamel mineralization is just beginning to be elucidated, new studies suggest that these proteins might have functions outside enamel formation. Furthermore, extracts of enamel proteins are currently being used to regenerate periodontal tissues destroyed by periodontal disease and new studies suggest that they might have chondrogenic and osteogenic properties. These new functions of enamel proteins will be the focus of this review.

Amelogenins: assembly, processing and control of crystal morphology

The remarkable properties of enamel crystals and their arrangements in an extraordinary micro-architecture are clear indications that the processes of crystal nucleation and growth in the extracellular matrix are highly controlled. The major extracellular events involved in enamel formation are: (a) delineation of space by the secretory ameloblasts and the dentino-enamel junction; (b) self-assembly of amelogenin proteins to form the supramolecular structural framework; (c) transportation of calcium and phosphate ions by the ameloblasts resulting in a supersaturated solution; (d) nucleation of apatite crystallites; and (e) elongated growth of the crystallites. Finally, during the 'maturation' step, rapid growth and thickening of the crystallites take place, which is concomitant with progressive degradation and eventual removal of the enamel extracellular matrix components (mainly amelogenins). This latter stage during which physical hardening of enamel occurs is perhaps unique to dental enamel. We have focused our in vitro studies on three major extracellular events: matrix assembly, matrix processing and control of crystal growth. This paper summarizes current knowledge on the assembly, processing and effect on crystal morphology by amelogenin proteins. The correlation between these three events and putative functional roles for amelogenin protein are discussed.

Degradative changes in whole enamel homogenates incubated in vitro in the presence of low calcium ion concentrations

The purpose of this study was to investigate overall degradative changes occurring to enamel matrix proteins in small, freeze-dried pieces of rat incisor enamel homogenized and incubated directly for 0-48 hours in a synthetic enamel fluid solution (165 mM total ionic strength with 0.153 mM calcium chloride) versus other samples homogenized and incubated for the same time intervals in distilled water. The results indicated that many alterations in the apparent molecular weights of enamel matrix proteins took place under both conditions although the rates for many degradative changes over a 48 hour period were often slower in distilled water than in synthetic enamel fluid. Freeze-dried enamel samples homogenized and incubated in 165 mM Tris-HCl buffer at pH 8.0 showed changes comparable to those seen with distilled water. This suggested that differences observed between samples incubated in enamel fluid versus distilled water were unrelated to pH or ionic strength of the solutions and may be the result of a requirement by some enamel proteinases for small amounts of free calcium ions in incubation media. Of interest were findings that some enamel matrix proteins, especially those in strips taken from the first half of the secretory stage of amelogenesis, were degraded much faster in distilled water than in synthetic enamel fluid. The reasons for this effect are unclear although, in this case, calcium ions could be inhibitory to hydrolysis of certain matrix proteins by the enamel proteinases.

Localized infusion of tunicamycin in rat hemimandibles: alteration of the basal lamina associated with maturation stage ameloblasts

At the beginning of the maturation stage of amelogenesis, ameloblasts deposit a basal lamina (BL) at the interface between their apical surface and maturing enamel. This structure is rich in glycoconjugates and is proposed to exhibit adhesive and/or filtering functions. To clarify its role, we have applied a recently developed surgical window model to locally administer tunicamycin (TM), an antibiotic that interferes with N-glycosylation, in the rat hemimandible using an osmotic minipump. Male Wistar rats were infused with either TM or saline as a control. Lectin-gold cytochemistry was performed to reveal glycoconjugates in the BL. Immunogold labeling of enamel proteins and albumin was carried out to verify whether depletion of N-linked sugars in the BL affects the content and distribution of endogenous and exogenous proteins in the enamel layer. Under the influence of the drug, the BL became irregular and exhibited alterations in structural organization and composition. The number of Helix pomatia agglutinin binding sites was not significantly affected but their distribution was altered. The labeling density of wheat germ agglutinin over the BL was slightly reduced. Immunoreactivity for enamel proteins showed only a small decrease, but that of albumin, both between ameloblasts and within the enamel layer, increased significantly. No structural alterations were observed in the contralateral incisor and in other sampled tissues and organs. These results demonstrate that it is possible to achieve a localized administration of TM without systemic side effects and lend support to the proposal that the BL represents a specialized structure with filtering functions.(J Histochem Cytochem 49:165-176, 2001)

Mass spectrometry of native rat amelogenins: primary transcripts, secretory isoforms, and C-terminal degradation

Cloning technologies have established unambiguously that amelogenins always seem larger in molecular weight (Mr) by gel electrophoresis (SDS-PAGE) than by mass spectrometry (MS). This has caused many problems relating cloned versions of amelogenin to proteins actually secreted by ameloblasts in vivo. In this study, discrete protein fractions at 31-20 kDa (Mr(SDS)) were prepared from freeze-dried rat incisor enamel by techniques optimized for preserving protein integrity. N-terminal sequence and amino acid compositional analyses indicated that the major protein forming these fractions was amelogenin. As expected, the molecular weights estimated by matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ESI) MS were significantly less than their apparent molecular weights estimated by SDS-PAGE. Plots of Mr(SDS) vs. Mr(MS) for all fractions showed high linear correlation (r = 0.992). Analysis of MS data further indicated that the major protein in the 27-kDa fraction corresponded to the R180 secretory isoform of rat amelogenin, whereas some minor proteins in the 23-kDa fraction likely corresponded to a R156 secretory isoform. This was in contrast to major proteins forming the 25-, 24-, and 23-kDa fractions (Mr(SDS)), which seemed to represent proteolytic fragments of R180 progressively altered at the P169-A170, P164-L165, and F151-S152 C-terminal cleavage sites, respectively. Proteins in the 20-kDa fraction (Mr(SDS)) most closely matched by ESI-MS fragments of the R156 secretory isoform that were C-terminally-modified at the equivalent P164-L165 site.

The structural biology of the developing dental enamel matrix

The biomineralization of the dental enamel matrix with a carbonated hydroxyapatite mineral generates one of the most remarkable examples of a vertebrate mineralized tissue. Recent advances in the molecular biology of ameloblast gene products have now revealed the primary structures of the principal proteins involved in this extracellular mineralizing system, amelogenins, tuftelins, ameloblastins, enamelins, and proteinases, but details of their secondary, tertiary, and quaternary structures, their interactions with other matrix and or cell surface proteins, and their functional role in dental enamel matrix mineralization are still largely unknown. This paper reviews our current knowledge of these molecules, the probable molecular structure of the enamel matrix, and the functional role of these extracellular matrix proteins. Recent studies on the major structural role played by the amelogenin proteins are discussed, and some new data on synthetic amelogenin matrices are reviewed.

Differences in the pattern of lanthanum diffusion into predentine and dentine in mouse incisors and molars

Lanthanum nitrate was either perfused intravascularly or segments of mouse tooth were immersed in a fixative solution containing the tracer. The tracer deposits were examined in young (8-day-old) and older (8-week-old) mouse incisors and molars, demineralized or undemineralized. Lanthanum passed the distal junctional complex of odontoblasts and appeared in the predentine of incisors as large electron-dense stellate aggregates, 40-70 nm in diameter, and in molars as round, 20-40 nm dots. In dentine, tracer deposits were detected at three locations. Near the predentine dentine junction, the tracer densely stained a band 0.5-2.5 microm in width, also termed metadentine; in the inner circumpulpal dentine, the staining was weaker or lacking in an area extending 5-7 microm from the predentine-dentine junction; in outer circumpulpal dentine, lateral diffusion had occurred in porosities of intertubular dentine. Lanthanum impregnated the walls of dentine tubules and a peritubular-like dentine. In contrast, the mantle dentine was never stained. These differences in the pattern of diffusion prove that lanthanum staining is age-dependent and varies between mouse incisors and molars, independently of tissue processing. Architectural properties and driving flux are involved in the transport and localization of lanthanum in predentine and dentine.

Tyrosyl motif in amelogenins binds N-acetyl-D-glucosamine

Ameloblasts secrete amelogenins on the pre-existing enamel matrix glycoproteins at the dentine-enamel junction. The hypothesis that amelogenins may interact with enamel matrix glycoproteins is tested by hemagglutination of purified, native (porcine) and recombinant murine amelogenins (rM179 and rM166) and hemagglutination inhibition with sugars. Amelogenin agglutination of murine erythrocytes was specifically inhibited by N-acetylglucosamine (GlcNAc), chitobiose, and chitotetraose and by ovalbumin with terminal GlcNAc. The GlcNAc affinity was confirmed by dosimetric binding of rM179 with [14C]GlcNAc, specific binding in relation to varying concentrations of GlcNAc, Scatchard plot analysis and competitive inhibition with cold GlcNAc. The hemagglutination activity and [14C]GlcNAc affinity were retained by the NH2-terminal tyrosine-rich amelogenin peptide (TRAP) but not by the leucine-rich amelogenin peptide, LRAP (a polypeptide sharing 33 amino acid residues of TRAP), or by the C-terminal 13 residue polypeptide of amelogenin (rM179). Since TRAP but not the 33-residue sequence of the TRAP shared by LRAP bound to [14C]GlcNAc, we inferred that the GlcNAc binding motif was located in the 13-residue tyrosyl C-terminal domain of TRAP (PYPSYGYEPMGGW), which was absent from LRAP. [14C]GlcNAc did indeed bind to this "amelogenin tyrosyl motif peptide" but not when the tyrosyl residues were substituted with phenylalanine or when the third proline was replaced by threonine. Significantly, this latter modification mimics a point mutation identified in a case of human X-linked amelogenesis imperfecta. The amelogenin tyrosyl motif peptide sequence showed a similarity to the secondary GlcNAc-binding site of wheat germ agglutinin.

The use of autoradiography and cycloheximide to determine the origin of enamel proteins in the maturation ameloblasts of the rat incisor

Morphological and cytochemical studies have suggested that maturation ameloblasts participate in protein loss by absorbing and degrading enamel proteins as the enamel matures. Several immunocytochemical and autoradiographic studies have suggested other possible explanations for the presence of enamel matrix proteins in maturation ameloblasts. The weakness of these autoradiographic studies is the uncontrolled distribution of systemically injected radioactive amino acids, making it impossible to trace the source of the visualized intracellular isotope. This study used a localized technique to control the targets of the applied isotope and to identify enamel matrix proteins in the maturation ameloblasts with more confidence about their origin. The amount of labelled enamel protein was higher in maturation ameloblasts than transitional ameloblasts. When cycloheximide, a protein-synthesis inhibitor, was applied, there was no effect on the amount of labelled protein in the maturation ameloblasts. These findings support the hypothesis that maturation ameloblasts actively resorb and degrade enamel matrix proteins during enamel formation in the mandibular incisor of the rat.

Comparative immunochemical analyses of the developmental expression and distribution of ameloblastin and amelogenin in rat incisors

Mineralized tissues are unique in using proteins to attract and organize calcium and phosphate ions into a structured mineral phase. A precise knowledge of the expression and extracellular distribution of matrix proteins is therefore very important in understanding their function. The purpose of this investigation was to obtain comparative information on the expression, intracellular and extracellular distribution, and dynamics of proteins representative of the two main classes of enamel matrix proteins. Amelogenins were visualized using an antibody and an mRNA probe prepared against the major alternatively spliced isoform in rodents, and nonamelogenins by antibodies and mRNA probes specific to one enamel protein referred to by three names: ameloblastin, amelin, and sheathlin. Qualitative and quantitative immunocytochemistry, in combination with immunoblotting and in situ hybridization, indicated a correlation between mRNA signal and sites of protein secretion for amelogenin, but not for ameloblastin, during the early presecretory and mid- to late maturation stages, during which mRNA signals were detected but no proteins appeared to be secreted. Extracellular amelogenin immunoreactivity was generally weak near secretory surfaces, increasing over a distance of about 1.25 microm to reach a level slightly above an amount expected if the protein were being deposited evenly across the enamel layer. Immunolabeling for ameloblastin showed an inverse pattern, with relatively more gold particles near secretory surfaces and much fewer deeper into the enamel layer. Administration of brefeldin A and cycloheximide to stop protein secretion revealed that the immunoblotting pattern of amelogenin was relatively stable, whereas ameloblastin broke down rapidly into lower molecular weight fragments. The distance from the cell surface at which immunolabeling for amelogenin stabilized generally corresponded to the point at which that for ameloblastin started to show a net reduction. These data suggest a correlation between the distribution of amelogenin and ameloblastin and that intact ameloblastin has a transient role in promoting/stabilizing crystal elongation. (J Histochem Cytochem 46:911-934, 1998)

Cellular and chemical events during enamel maturation

This review focuses on the process of enamel maturation, a series of events associated with slow, progressive growth in the width and thickness of apatitic crystals. This developmental step causes gradual physical hardening and transformation of soft, newly formed enamel into one of the most durable mineralized tissues produced biologically. Enamel is the secretory product of specialized epithelial cells, the ameloblasts, which make this covering on the crowns of teeth in two steps. First, they roughly "map out" the location and limits (overall thickness) of the entire extracellular layer as a protein-rich, acellular, and avascular matrix filled with thin, ribbon-like crystals of carbonated hydroxyapatite. These initial crystals are organized spatially into rod and interrod territories as they form, and rod crystals are lengthened by Tomes' processes in tandem with appositional movement of ameloblasts away from the dentin surface. Once the full thickness of enamel has been formed, ameloblasts initiate a series of repetitive morphological changes at the enamel surface in which tight junctions and deep membrane infoldings periodically appear (ruffle-ended), then disappear for short intervals (smooth-ended), from the apical ends of the cells. As this happens, the enamel covered by these cells changes rhythmically in net pH from mildly acidic (ruffle-ended) to near-physiologic (smooth-ended) as mineral crystals slowly expand into the "spaces" (volume) formerly occupied by matrix proteins and water. Matrix proteins are processed and degraded by proteinases throughout amelogenesis, but they undergo more rapid destruction once ameloblast modulation begins. Ruffle-ended ameloblasts appear to function primarily as a regulatory and transport epithelium for controlling the movement of calcium and other ions such as bicarbonate into enamel to maintain buffering capacity and driving forces optimized for surface crystal growth. The reason ruffle-ended ameloblasts become smooth-ended periodically is unknown, although this event seems to be crucial for sustaining long-term crystal growth.

Synthesis, secretion, degradation, and fate of ameloblastin during the matrix formation stage of the rat incisor as shown by immunocytochemistry and immunochemistry using region-specific antibodies

Rat ameloblastin is a recently cloned tooth-specific enamel matrix protein containing 422 amino acid residues. We investigated the expression of this protein during the matrix formation stage of the rat incisor immunohistochemically and immunochemically, using anti-synthetic peptide antibodies that recognize residues 27-47 (Nt), 98-107 (M-1), 224-232 (M-2), 386-399 (M-3), and 406-419 (Ct) of ameloblastin. Immunohistochemical preparations using antibodies Nt and M-1 stained the Golgi apparatus and secretory granules of the secretory ameloblast and the entire thickness of the enamel matrix. Only M-1 intensely stained the peripheral region of the enamel rods. Immunostained protein bands were observed near 65, 55, and below 22 kD. Immunohistochemical preparations using antibodies M-2 and Ct stained the Golgi apparatus and secretory granules of the ameloblast and the immature enamel adjacent to the secretion sites, but not deeper enamel layers. Immunostaining using M-2 and Ct revealed protein bands near 65 and 40-56 kD, and 65, 55, 48, 36, and 25 kD, respectively. M-3 stained the cis side of the Golgi apparatus but not the enamel matrix. This antibody recognized a protein band near 55 kD, but none larger. After brefeldin A treatment, immunoreaction of the 55-kD protein band intensified, and dilated cisternae of rER of the secretory ameloblast contained immunoreactive material irrespective of the antibodies used. These data indicate that ameloblastin is synthesized as a 55-kD core protein and then is post-translationally modified with O-linked oligosaccharides to become the 65-kD secretory form. Initial cleavages of the 65-kD protein generate N-terminal polypeptides, some of which concentrate in the prism sheath, and C-terminal polypeptides, which are rapidly degraded and lost from the enamel matrix soon after secretion.

Immunodetection of enamel- and cementum-related (bone) proteins at the enamel-free area and cervical portion of the tooth in rat molars

Enamel and dentin at the cervical portion of the tooth are frequently covered by a collagen-free matrix referred to as acellular afibrillar cementum (AAC). It is believed that AAC deposition occurs when the enamel organ is displaced or disrupted, and mesenchymal cells from the dental follicle gain access to the tooth surface, differentiate into cementoblasts, and secrete noncollagenous proteins typically found in collagen-based mineralized tissues. A similar thin layer of mineralized matrix is found at the enamel-free area (EFA) of rodent molars, but in this case the matrix is covered by inner enamel epithelium (IEE) throughout development. We have, therefore, used this site as a paradigm to test the hypothesis that typical mesenchymal matrix proteins can also be found in association with epithelial cells. To this end, we have analyzed the presence and distribution of enamel- and cementum-related matrix proteins at the EFA and at the cervical portion of the tooth. Rat mandibular molars were processed for colloidal gold immunolabeling with antibodies to amelogenins, bone sialoprotein (BSP), osteopontin (OPN), osteocalcin (OC), and dentin sialoprotein (DSP), and the plasma proteins alpha 2 HS-glycoprotein and albumin. The EFA matrix was immunoreactive for amelogenins as well as for BSP, OPN, OC, and alpha 2 HS-glycoprotein, but not for albumin and DSP. The AAC was, similar to the EFA matrix, labeled for BSP, OPN, OC, and alpha 2 HS-glycoprotein. These data show for the first time that the EFA matrix is comprised of a mixture of enamel- and cementum-related proteins, a situation that parallels the distribution of matrix constituents at the cervical portion of the tooth. Since the EFA matrix is deposited on top of the mineralized dentin, and since the enamel organ seals off the forming matrix, it is concluded that EFA cells are responsible for the production of these proteins. Consistent with previous reports showing that epithelial cells can produce both BSP and OPN in some circumstances, the data also suggest that AAC may be deposited by cells of epithelial origin. Furthermore, they lend support to the possibility that cells derived from Hertwig's epithelial root sheath may likewise be capable of producing cementum matrix proteins.

Tuftelin: enamel mineralization and amelogenesis imperfecta

Tuftelin is a novel acidic enamel protein thought to play a major role in enamel mineralization. Its identity and localization has been confirmed by amino acid composition, enzyme-linked immunosorbant assay, Western blots, indirect immunohistochemistry and high resolution protein-A gold immunocytochemistry. The deduced tuftelin protein (pI 5.2) contains 389 amino acids and has a calculated peptide molecular mass of 43,814 Da. Immunological studies suggest conservation of tuftelin structure between species throughout vertebrate evolution. The cDNA sequence encodes for several putative post-translation sites including one N-glycosylation consensus site, seven O-glycosylation sites and seven phosphorylation sites, as well as an EF-hand calcium-binding domain (with mismatch), localized towards the N-terminal region. At the C-terminal region (residues 252-345) tuftelin contains structurally relevant determinants for self assembly. We recently cloned and partially sequenced the human tuftelin gene (four exons have now been sequenced). These sequences include exon 1 and over 1000 bases of the putative promoter region. Employing fluorescent in situ hybridization, we mapped the human tuftelin gene to chromosome 1q 21-31. Localization of the human tuftelin gene to a well-defined cytogenetic region may be important in understanding the aetiology of autosomally inherited amelogenesis imperfecta, the most common enamel hereditary disease.

Transient accumulation of proteins at interrod and rod enamel growth sites

Conceptually, there should be a brief interval in time when newly secreted proteins "pile up" at secretory sites just outside the membrane of ameloblasts. Indeed, previous cytochemical studies have suggested that glycosylated and/or sulfated glycoproteins accumulate at enamel growth sites. Colloidal gold lectin cytochemistry and immunocytochemistry with antibodies to enamel proteins and phosphoserine, combined with cycloheximide and brefeldin A to inhibit protein synthesis and secretion, were applied to characterize the distribution of newly formed proteins at enamel interrod and rod growth sites. Although enamel growth sites show a "rarefied" appearance, the results indicate that one or more subclasses of enamel proteins accumulate near the cell surface at sites where elongation of enamel crystallites contributes to thickening of the enamel layer. These proteins are glycosylated and/or phosphorylated and, at least in the case of the glycosylated ones, are rapidly processed after they are released extracellularly. In contrast, immunolabeling for amelogenins is generally weaker near the cell surface and more intense at a short distance away from the site where crystallites elongate. The data suggest that the enamel proteins accumulating at growth sites likely belong to the non-amelogenin category and play a transient role in promoting the lengthening of crystallites. It is concluded that areas near the ameloblast membrane where certain enamel proteins accumulate in fact constitute the equivalent of a mineralization front.

Developmental changes in the pH of enamel fluid and its effects on matrix-resident proteinases

The objectives of this study were to measure pH in developing enamel at progressively older (more mature) stages of amelogenesis in vivo, and then to formulate synthetic enamel fluid mixtures that approximated these pH values for in vitro studies. The ultimate goal was to characterize the molecular weights of proteinases visualized by enzymograms incubated in synthetic enamel fluid using gelatin and casein as substrates. For most experiments, the proteinases were extracted en masse from small freeze-dried enamel strips directly into a non-reducing sample preparation buffer. In some experiments, we pre-treated the enamel strips with acetic acid to determine if this common method for demineralization and protein extraction caused any changes in the activity levels of the enamel proteinases. In other experiments, we first soaked enamel strips in synthetic enamel fluid to determine solubility of the proteinases within an aqueous phase. The results indicated that the pH of developing enamel remained fairly constant near pH 7.23 across the secretory stage, but it was generally more acidic (6.93) and fluctuated in focal areas between mildly acidic (6.2-6.8) and near-neutral (7.2) conditions across the maturation stage. The pH then slowly rose to near 7.35 when the enamel was almost mature (hard). The acidic conditions were generally inhibitory to most enamel proteinases, but there were some caseinase activities in mid-maturation-stage enamel near 23-30 kDa which appeared to be activated by weakly acidic conditions (pH 6.28). Pre-treatment of enamel samples with 0.5 M acetic acid markedly altered the overall profile of enamel proteinases, causing activation of some latent proteinase activities and permanent inhibition of other activities. Most proteinases in whole homogenates were insoluble in synthetic enamel fluid. This suggests that they may be tightly bound, directly or indirectly, to matrix proteins or mineral components in situ.

Protein dynamics of amelogenesis

BACKGROUND

The synthesis, secretion, and fate of matrix proteins released by ameloblasts during enamel formation was studied in continuously erupting rat incisors.

METHODS

Computerized image processing was used to quantify silver grain distribution in radioautographs of sections prepared from rats injected with 3H-methionine, and this was correlated with fluorographs defining radiolabeling patterns of proteins in enamel organ cell and enamel homogenates prepared from freeze-dried teeth of rats injected with 35S-methionine and other radioactive amino acids and precursors such as sugar, sulfate, and phosphate. Some rats were also treated with brefeldin A to characterize newly formed proteins blocked from being secreted from ameloblasts.

RESULTS

The results indicate that ameloblasts rapidly synthesize and secrete (minutes) at least five primary enamel matrix proteins, including a 65 kDa sugar-containing sulfated enamel protein and four nonsulfated proteins with molecular weights near 31, 29, 27, and 23 kDa as estimated by SDS-PAGE. The 27 kDa protein appears to correspond to the primary amelogenin described in many species. The cells also appear to release at least one phosphoprotein with molecular weight near 27 kDa, which may be an amelogenin, and up to five cysteine-containing proteins with molecular weights near 94, 90, 72, 55, and 27 kDa. The proteins collectively are released at interrod and rod growth sites where they appear to remain close to their point of release from ameloblasts. The 65 kDa sulfated protein and 31 kDa nonsulfated protein are rapidly converted into lower molecular weight forms (hours), whereas nonsulfated proteins near 29, 27, and 23 kDa are more slowly transformed into fragments near 20, 18, and 10 kDa in molecular weight (days). These fragments do not accumulate but appear to be removed from the enamel layer as they are created.

CONCLUSIONS

Enamel proteins seen by Coomassie blue (or silver) staining of one-dimensional polyacrylamide gels, therefore, represent a composite image of newly secreted and derived forms of sulfated and nonsulfated proteins that sometimes have similar molecular weights.

Matrix proteins and mineralization: an overview

There is a wealth of information on the mineral and matrix components in bones and teeth, in the exoskeletons of invertebrates, and in dystrophic calcific deposits. This information includes detailed characterization of their physical and chemical composition and details on the gene localization and regulation of gene expression for the major and minor protein constituents. The reason that mineral deposition occurs in some tissues and not in others remains unclear. In this review, studies in solution, cell culture studies, and investigations in mutant animals will be surveyed to indicate which matrix proteins may affect mineralization. Most of the molecules that appear to be involved in initiation and regulation of biological mineral formation are anionic; they have structural features that facilitate interaction with mineral, cells, and other matrix molecules, and they can have more than one function. Despite extensive data it is not yet clear which of these molecules is absolutely essential for physiologic calcification in each of the calcified tissues.

Spatial distribution of enamel proteins and fibronectin at early stages of rat incisor tooth formation

Enamel proteins are secreted very early during amelogenesis, that is prior to mantle dentine formation, raising the possibility that they may participate in epithelial-mesenchymal interactions taking place during tooth development. These first enamel proteins associate with elements of the basement membrane interposed between the differentiating ameloblasts and odontoblasts. Fibronectin, a component of the basement membrane, is redistributed and accumulates along the apical portion of odontoblasts during their terminal differentiation. In order to determine whether any correlation exists between the redistribution of fibronectin and the secretion of the first enamel proteins, the spatial distribution of these two extracellular matrix proteins was examined during the presecretory stage of amelogenesis. Male Wistar rats were perfused with a formaldehyde-based fixative, and undemineralized and EDTA demineralized incisors were dehydrated in methanol and embedded in Lowicryl K4M resin. Ultrathin tissue sections were then processed for post-embedding, colloidal-gold immunocytochemistry with antibodies to enamel proteins, fibronectin or type III collagen. In the region of ameloblasts facing pulp, labelling for fibronectin was weak and mostly associated with the lamina fibroreticularis of the basement membrane separating differentiating ameloblasts and odontoblasts. As the mantle predentine formed the immunoreaction for fibronectin increased, particularly in the region of the basement membrane. Enamel proteins were also immunodetected in association with the lamina fibroreticularis and gradually accumulated as patches within mantle dentine and at its interface with ameloblasts. Von Korff collagen bundles, present between odontoblasts and in dentine, were immunolabelled for fibronectin and for type III collagen. Patches of granular material, immunoreactive for fibronectin and/or enamel proteins, were found along the odontoblastic processes and cell bodies. Although no evidence was obtained indicating a precise colocalization of fibronectin and enamel proteins, the results confirm that these two proteins can be found within similar extracellular compartments during mantle predentine-dentine formation. These data suggest that enamel proteins, by themselves or synergistically with other proteins, may play a part in the differentiation and/or formative events taking place at the ameloblast-odontoblast interface during the early stages of tooth development.