Determination of protein regions responsible for interactions of amelogenin with CD63 and LAMP1

Ameloblastin and its multifunctionality in amelogenesis: A review

Extracellular matrix proteins play crucial roles in the formation of mineralized tissues like bone and teeth via multifunctional mechanisms. In tooth enamel, ameloblastin (Ambn) is one such multifunctional extracellular matrix protein implicated in cell signaling and polarity, cell adhesion to the developing enamel matrix, and stabilization of prismatic enamel morphology. To provide a perspective for Ambn structure and function, we begin this review by describing dental enamel and enamel formation (amelogenesis) followed by a description of enamel extracellular matrix. We then summarize the established domains and motifs in Ambn protein, human amelogenesis imperfecta cases, and genetically engineered mouse models involving mutated or null Ambn. We subsequently delineate in silico, in vitro, and in vivo evidence for the amphipathic helix in Ambn as a proposed cell-matrix adhesive and then more recent in vitro evidence for the multitargeting domain as the basis for dynamic interactions of Ambn with itself, amelogenin, and membranes. The multitargeting domain facilitates tuning between Ambn-membrane interactions and self/co-assembly and supports a likely overall role for Ambn as a matricellular protein. We anticipate that this review will enhance the understanding of multifunctional matrix proteins by consolidating diverse mechanisms through which Ambn contributes to enamel extracellular matrix mineralization.

Leucine rich amelogenin peptide prevents ovariectomy-induced bone loss in mice

Amelogenins, major extra cellular matrix proteins of developing tooth enamel, are predominantly expressed by ameloblasts and play significant roles in the formation of enamel. Recently, amelogenin has been detected in various epithelial and mesenchymal tissues, implicating that it might have distinct functions in various tissues. We have previously reported that leucine rich amelogenin peptide (LRAP), one of the alternate splice forms of amelogenin, regulates receptor activator of NF-kappa B ligand (RANKL) expression in cementoblast/periodontal ligament cells, suggesting that the amelogenins, especially LRAP, might function as a signaling molecule in bone metabolism. The objective of this study was to identify and define LRAP functions in bone turnover. We engineered transgenic (TgLRAP) mice using a murine 2.3kb α1(I)-collagen promoter to drive expression of a transgene consisting of LRAP, an internal ribosome entry site (IRES) and enhanced green fluorescent protein (EGFP) to study functions of LRAP in bone formation and resorption. Calvarial cell cultures from the TgLRAP mice showed increased alkaline phosphatase (ALP) activity and increased formation of mineralized nodules compared to the cells derived from wild-type (WT) mice. The TgLRAP calvarial cells also showed an inhibitory effect on osteoclastogenesis in vitro. Gene expression comparison by quantitative polymerase chain reaction (Q-PCR) in calvarial cells indicated that bone formation makers such as Runx2, Alp, and osteocalcin were increased in TgLRAP compared to the WT cells. Meanwhile, Rankl expression was decreased in the TgLRAP cells in vitro. The ovariectomized (OVX) TgLRAP mice resisted bone loss induced by ovariectomy resulting in higher bone mineral density in comparison to OVX WT mice. The quantitative analysis of calcein intakes indicated that the ovariectomy resulted in increased bone formation in both WT and TgLRAP mice; OVX TgLRAP appeared to show the most remarkably increased bone formation. The parameters for bone resorption in tissue sections showed increased number of osteoclasts in OVX WT, but not in OVX TgLRAP over that of sham operated WT or TgLRAP mice, supporting the observed bone phenotypes in OVX mice. This is the first report identifying that LRAP, one of the amelogenin splice variants, affects bone turnover in vivo.

Epitope mapping of anti-amelogenin IgG in untreated celiac children

Children with untreated celiac disease (CeD) may develop enamel defects, and children with severe CeD have significantly increased levels of IgG to amelogenin, which may interfere with normal amelogenesis depending on which epitope(s) they bind. Children with untreated CeD (n = 42), for whom CeD had been confirmed either by biopsy (n = 17, cohort 1) or by the presence of particularly high serum levels of anti-transglutaminase 2 (TG2) IgA (n = 25, cohort 2), were selected from 146 children with CeD, and 10 controls were selected from 34 children who did not have CeD. Samples from these 52 children were used for detailed IgG anti-amelogenin, X isoform (AMELX) epitope mapping using 31 overlapping, 10-22mer peptides in ELISA. Although sera from both groups showed reactivity to peptides containing sequences from the N and C terminus of AMELX, sera from children with CeD reacted more strongly to peptides from the central region (amino acids 75-150) containing both a binding site for transforming growth factor-β (TGF-β), as well as the enzymatic cleavage sites for matrix metalloproteinase-20 and for kallikrein-4. Antigen-specific extraction revealed that only IgG to the central region cross-reacted to gliadin. Thus, cross-reactive anti-gliadin/amelogenin IgG may affect normal amelogenesis by interfering with enzymatic degradation, proper folding, and/or TGF-β signaling in children with untreated CeD.

Amelogenin Downregulates Interferon Gamma-Induced Major Histocompatibility Complex Class II Expression Through Suppression of Euchromatin Formation in the Class II Transactivator Promoter IV Region in Macrophages

Enamel matrix derivatives (EMDs)-based periodontal tissue regenerative therapy is known to promote healing with minimal inflammatory response after periodontal surgery, i. e., it promotes wound healing with reduced pain and swelling. It has also been reported that macrophages stimulated with amelogenin, a major component of EMD, produce various anti-inflammatory cytokines and growth factors. We previously found that stimulation of monocytes with murine recombinant M180 (rM180) amelogenin suppresses major histocompatibility complex class II (MHC II) gene expression using microarray analysis. However, the detailed molecular mechanisms for this process remain unclear. In the present study, we demonstrated that rM180 amelogenin selectively downmodulates the interferon gamma (IFNγ)-induced cell surface expression of MHC II molecules in macrophages and this mechanism mediated by rM180 appeared to be widely conserved across species. Furthermore, rM180 accumulated in the nucleus of macrophages at 15 min after stimulation and inhibited the protein expression of class II transactivator (CIITA) which controls the transcription of MHC II by IFNγ. In addition, reduced MHC II expression on macrophages pretreated with rM180 impaired the expression of T cell activation markers CD25 and CD69, T cell proliferation ability, and IL-2 production by allogenic CD4⁺ T lymphocytes in mixed lymphocyte reaction assay. The chromatin immunoprecipitation assay showed that IFNγ stimulation increased the acetylation of histone H3 lysine 27, which is important for conversion to euchromatin, as well as the trimethylation of histone H3 lysine 4 levels in the CIITA promoter IV (p-IV) region, but both were suppressed in the group stimulated with IFNγ after rM180 treatment. In conclusion, the present study shows that amelogenin suppresses MHC II expression by altering chromatin structure and inhibiting CIITA p-IV transcription activity, and attenuates subsequent T cell activation. Clinically observed acceleration of wound healing after periodontal surgery by amelogenin may be partially mediated by the mechanism elucidated in this study. In addition, the use of recombinant amelogenin is safe because it is biologically derived protein. Therefore, amelogenin may also be used in future as an immunosuppressant with minimal side effects for organ transplantation or MHC II-linked autoimmune diseases such as type I diabetes, multiple sclerosis, and rheumatoid arthritis, among others.

Novel LRAP-binding partner revealing the plasminogen activation system as a regulator of cementoblast differentiation and mineral nodule formation in vitro

Amelogenin isoforms, including full-length amelogenin (AMEL) and leucine-rich amelogenin peptide (LRAP), are major components of the enamel matrix, and are considered as signaling molecules in epithelial-mesenchymal interactions regulating tooth development and periodontal regeneration. Nevertheless, the molecular mechanisms involved are still poorly understood. The aim of the present study was to identify novel binding partners for amelogenin isoforms in the cementoblast (OCCM-30), using an affinity purification assay (GST pull-down) followed by mass spectrometry and immunoblotting. Protein-protein interaction analysis for AMEL and LRAP evidenced the plasminogen activation system (PAS) as a potential player regulating OCCM-30 response to amelogenin isoforms. For functional assays, PAS was either activated (plasmin) or inhibited (ε-aminocaproic acid [aminocaproic]) in OCCM-30 cells and the cell morphology, mineral nodule formation, and gene expression were assessed. PAS inhibition (EACA 100 mM) dramatically decreased mineral nodule formation and expression of OCCM-30 differentiation markers, including osteocalcin (Bglap), bone sialoprotein (Ibsp), osteopontin (Spp1), tissue-nonspecific alkaline phosphatase (Alpl) and collagen type I (Col1a1), and had no effect on runt-related transcription factor 2 (Runx2) and Osterix (Osx) mRNA levels. PAS activation (plasmin 5 µg/µl) significantly increased Col1a1 and decreased Bglap mRNA levels (p < .05). Together, our findings shed new light on the potential role of plasminogen signaling pathway in the control of the amelogenin isoform-mediated response in cementoblasts and provide new insights into the development of targeted therapies.

Ameloblastin Binds to Phospholipid Bilayers via a Helix-Forming Motif within the Sequence Encoded by Exon 5

Ameloblastin (Ambn), the most abundant non-amelogenin enamel protein, is intrinsically disordered and has the potential to interact with other enamel proteins and with cell membranes. Here, through multiple biophysical methods, we investigated the interactions between Ambn and large unilamellar vesicles (LUVs), whose lipid compositions mimicked cell membranes involved in epithelial cell-extracellular matrix adhesion. Using a series of Ambn Trp/Phe variants and Ambn mutants, we further showed that Ambn binds to LUVs through a highly conserved motif within the sequence encoded by exon 5. Synthetic peptides derived from different regions of Ambn confirmed that the sequence encoded by exon 5 is involved in LUV binding. Sequence analysis of Ambn across different species showed that the N-terminus of this sequence contains a highly conserved motif with a propensity to form an amphipathic helix. Mutations in the helix-forming sequence resulted in a loss of peptide binding to LUVs. Our in vitro data suggest that Ambn binds the lipid membrane directly through a conserved helical motif and have implications for biological events such as Ambn-cell interactions, Ambn signaling, and Ambn secretion via secretory vesicles.

Potential function of TGF-β isoforms in maturation-stage ameloblasts

OBJECTIVES

To investigate potential functions of transforming growth factor-beta (TGF-β) isoforms in maturation-stage ameloblasts during amelogenesis.

METHODS

In vivo activation of TGF-β was characterized by using matrix metalloproteinase 20 null (Mmp20^-/-) and wild-type (Mmp20^+/+) mice. Using mHAT9d cells cultured in the presence of each TGF-β isoform, (1) cell proliferation was determined by MTS assay, (2) immunostaining with anti-cleaved caspase-3 monoclonal antibody was performed and apoptotic indices were measured, (3) gene expression was analyzed by RT-qPCR, and (4) the uptake of amelogenin into mHAT9d cells was directly observed using a fluorescence microscope.

RESULTS

TGF-β1 and TGF-β3 were present in the enamel matrix of developing teeth which were activated by MMP20 in vivo. A genetic study revealed that the three TGF-β isoforms upregulate kallikrein 4 (KLK4) mRNA levels but downregulate carbonic anhydrase II. Moreover, TGF-β1 and TGF-β2 significantly upregulated the mRNA level of amelotin, whereas TGF-β3 dramatically downregulated the mRNA levels of odontogenic ameloblast-associated protein (ODAM), family with sequence similarity 83 member H (FAM83H), and alkaline phosphatase (ALP). Immunostaining analysis showed that the apoptosis of mHAT9d cells is induced by three TGF-β isoforms, with TGF-β3 being most effective. Both TGF-β1 and TGF-β3 induced endocytosis of amelogenin.

CONCLUSIONS

We propose that TGF-β is regulated in an isoform-specific manner to perform multiple biological functions such as gene expression related to the structure of basal lamina/ameloblasts, mineral ion transport, apoptosis, and endocytosis in maturation-stage ameloblasts.

Molecular identification of disk abalone (Haliotis discus discus) tetraspanin 33 and CD63: Insights into potent players in the disk abalone host defense system

Tetraspanins are a superfamily of transmembrane proteins involved in a diverse range of physiological processes including differentiation, adhesion, signal transduction, cell motility, and immune responses. In the present study, two tetraspanins, CD63 and tetraspanin 33 (TSPAN33) from disk abalone (AbCD63 and AbTSPAN33), were identified and characterized at the molecular level. The coding sequences for AbCD63 and AbTSPAN33 encoded polypeptides of 234 and 290 amino acids (aa) with predicted molecular mass of 25.3 and 32.5 kDa, respectively. The deduced AbCD63 and AbTSPAN33 protein sequences were also predicted to have a typical tetraspanin domain architecture, including four transmembrane domains (TM), short N- and C- terminal regions, a short intracellular loop, as well as a large and small extracellular loop. A characteristic CCG motif and cysteine residues, which are highly conserved across CD63 and TSPAN33 proteins of different species, were present in the large extracellular loop of both abalone tetraspanins. Phylogenetic analysis revealed that the AbCD63 and AbTSPAN33 clustered in the invertebrate subclade of tetraspanins, thus exhibiting a close relationship with tetraspanins of other mollusks. The AbCD63 and AbTSPAN33 mRNA transcripts were detected at early embryonic development stages of disk abalone with significantly higher amounts at the trochophore stage, suggesting the involvement of these proteins in embryonic development. Both AbCD63 and AbTSPAN33 were ubiquitously expressed in all the tissues of unchallenged abalones analyzed, with the highest expression levels found in hemocytes. Moreover, significant induction of AbCD63 and AbTSPAN33 mRNA expression was observed in immunologically important tissues, such as hemocytes and gills, upon stimulation with live bacteria (Vibrio parahaemolyticus and Listeria monocytogenes), virus (viral hemorrhagic septicemia virus), and two potent immune stimulators [polyinosinic:polycytidylic acid (poly I:C) and lipopolysaccharide (LPS)]. Collectively, these findings suggest that AbCD63 and AbTSPAN33 are involved in innate immune responses in disk abalone during pathogenic stress.

DENTAL ENAMEL FORMATION AND IMPLICATIONS FOR ORAL HEALTH AND DISEASE

Dental enamel is the hardest and most mineralized tissue in extinct and extant vertebrate species and provides maximum durability that allows teeth to function as weapons and/or tools as well as for food processing. Enamel development and mineralization is an intricate process tightly regulated by cells of the enamel organ called ameloblasts. These heavily polarized cells form a monolayer around the developing enamel tissue and move as a single forming front in specified directions as they lay down a proteinaceous matrix that serves as a template for crystal growth. Ameloblasts maintain intercellular connections creating a semi-permeable barrier that at one end (basal/proximal) receives nutrients and ions from blood vessels, and at the opposite end (secretory/apical/distal) forms extracellular crystals within specified pH conditions. In this unique environment, ameloblasts orchestrate crystal growth via multiple cellular activities including modulating the transport of minerals and ions, pH regulation, proteolysis, and endocytosis. In many vertebrates, the bulk of the enamel tissue volume is first formed and subsequently mineralized by these same cells as they retransform their morphology and function. Cell death by apoptosis and regression are the fates of many ameloblasts following enamel maturation, and what cells remain of the enamel organ are shed during tooth eruption, or are incorporated into the tooth's epithelial attachment to the oral gingiva. In this review, we examine key aspects of dental enamel formation, from its developmental genesis to the ever-increasing wealth of data on the mechanisms mediating ionic transport, as well as the clinical outcomes resulting from abnormal ameloblast function.

MiR-153 Regulates Amelogenesis by Targeting Endocytotic and Endosomal/lysosomal Pathways-Novel Insight into the Origins of Enamel Pathologies

Amelogenesis imperfecta (AI) is group of inherited disorders resulting in enamel pathologies. The involvement of epigenetic regulation in the pathogenesis of AI is yet to be clarified due to a lack of knowledge about amelogenesis. Our previous genome-wide microRNA and mRNA transcriptome analyses suggest a key role for miR-153 in endosome/lysosome-related pathways during amelogenesis. Here we show that miR-153 is significantly downregulated in maturation ameloblasts compared with secretory ameloblasts. Within ameloblast-like cells, upregulation of miR-153 results in the downregulation of its predicted targets including Cltc, Lamp1, Clcn4 and Slc4a4, and a number of miRNAs implicated in endocytotic pathways. Luciferase reporter assays confirmed the predicted interactions between miR-153 and the 3'-UTRs of Cltc, Lamp1 (in a prior study), Clcn4 and Slc4a4. In an enamel protein intake assay, enamel cells transfected with miR-153 show a decreased ability to endocytose enamel proteins. Finally, microinjection of miR-153 in the region of mouse first mandibular molar at postnatal day 8 (PN8) induced AI-like pathologies when the enamel development reached maturity (PN12). In conclusion, miR-153 regulates maturation-stage amelogenesis by targeting key genes involved in the endocytotic and endosomal/lysosomal pathways, and disruption of miR-153 expression is a potential candidate etiologic factor contributing to the occurrence of AI.

Enamel ribbons, surface nodules, and octacalcium phosphate in C57BL/6 Amelx-/- mice and Amelx+/- lyonization

BACKGROUND

Amelogenin is required for normal enamel formation and is the most abundant protein in developing enamel.

METHODS

Amelx^+/+, Amelx^+/- , and Amelx^-/- molars and incisors from C57BL/6 mice were characterized using RT-PCR, Western blotting, dissecting and light microscopy, immunohistochemistry (IHC), transmission electron microscopy (TEM), scanning electron microscopy (SEM), backscattered SEM (bSEM), nanohardness testing, and X-ray diffraction.

RESULTS

No amelogenin protein was detected by Western blot analyses of enamel extracts from Amelx^-/- mice. Amelx^-/- incisor enamel averaged 20.3 ± 3.3 μm in thickness, or only 1/6th that of the wild type (122.3 ± 7.9 μm). Amelx^-/- incisor enamel nanohardness was 1.6 Gpa, less than half that of wild-type enamel (3.6 Gpa). Amelx^+/- incisors and molars showed vertical banding patterns unique to each tooth. IHC detected no amelogenin in Amelx^-/- enamel and varied levels of amelogenin in Amelx^+/- incisors, which correlated positively with enamel thickness, strongly supporting lyonization as the cause of the variations in enamel thickness. TEM analyses showed characteristic mineral ribbons in Amelx^+/+ and Amelx^-/- enamel extending from mineralized dentin collagen to the ameloblast. The Amelx^-/- enamel ribbons were not well separated by matrix and appeared to fuse together, forming plates. X-ray diffraction determined that the predominant mineral in Amelx^-/- enamel is octacalcium phosphate (not calcium hydroxyapatite). Amelx^-/- ameloblasts were similar to wild-type ameloblasts except no Tomes' processes extended into the thin enamel. Amelx^-/- and Amelx^+/- molars both showed calcified nodules on their occlusal surfaces. Histology of D5 and D11 developing molars showed nodules forming during the maturation stage.

CONCLUSION

Amelogenin forms a resorbable matrix that separates and supports, but does not shape early secretory-stage enamel ribbons. Amelogenin may facilitate the conversion of enamel ribbons into hydroxyapatite by inhibiting the formation of octacalcium phosphate. Amelogenin is necessary for thickening the enamel layer, which helps maintain ribbon organization and development and maintenance of the Tomes' process.

SLC26A Gene Family Participate in pH Regulation during Enamel Maturation

The bicarbonate transport activities of Slc26a1, Slc26a6 and Slc26a7 are essential to physiological processes in multiple organs. Although mutations of Slc26a1, Slc26a6 and Slc26a7 have not been linked to any human diseases, disruption of Slc26a1, Slc26a6 or Slc26a7 expression in animals causes severe dysregulation of acid-base balance and disorder of anion homeostasis. Amelogenesis, especially the enamel formation during maturation stage, requires complex pH regulation mechanisms based on ion transport. The disruption of stage-specific ion transporters frequently results in enamel pathosis in animals. Here we present evidence that Slc26a1, Slc26a6 and Slc26a7 are highly expressed in rodent incisor ameloblasts during maturation-stage tooth development. In maturation-stage ameloblasts, Slc26a1, Slc26a6 and Slc26a7 show a similar cellular distribution as the cystic fibrosis transmembrane conductance regulator (Cftr) to the apical region of cytoplasmic membrane, and the distribution of Slc26a7 is also seen in the cytoplasmic/subapical region, presumably on the lysosomal membrane. We have also examined Slc26a1 and Slc26a7 null mice, and although no overt abnormal enamel phenotypes were observed in Slc26a1-/- or Slc26a7-/- animals, absence of Slc26a1 or Slc26a7 results in up-regulation of Cftr, Ca2, Slc4a4, Slc4a9 and Slc26a9, all of which are involved in pH homeostasis, indicating that this might be a compensatory mechanism used by ameloblasts cells in the absence of Slc26 genes. Together, our data show that Slc26a1, Slc26a6 and Slc26a7 are novel participants in the extracellular transport of bicarbonate during enamel maturation, and that their functional roles may be achieved by forming interaction units with Cftr.

Genome-wide analysis of miRNA and mRNA transcriptomes during amelogenesis

BACKGROUND

In the rodent incisor during amelogenesis, as ameloblast cells transition from secretory stage to maturation stage, their morphology and transcriptome profiles change dramatically. Prior whole genome transcriptome analysis has given a broad picture of the molecular activities dominating both stages of amelogenesis, but this type of analysis has not included miRNA transcript profiling. In this study, we set out to document which miRNAs and corresponding target genes change significantly as ameloblasts transition from secretory- to maturation-stage amelogenesis.

RESULTS

Total RNA samples from both secretory- and maturation-stage rat enamel organs were subjected to genome-wide miRNA and mRNA transcript profiling. We identified 59 miRNAs that were differentially expressed at the maturation stage relative to the secretory stage of enamel development (False Discovery Rate (FDR)<0.05, fold change (FC)≥1.8). In parallel, transcriptome profiling experiments identified 1,729 mRNA transcripts that were differentially expressed in the maturation stage compared to the secretory stage (FDR<0.05, FC≥1.8). Based on bioinformatics analyses, 5.8% (629 total) of these differentially expressed genes (DEGS) were highlighted as being the potential targets of 59 miRNAs that were differentially expressed in the opposite direction, in the same tissue samples. Although the number of predicted target DEGs was not higher than baseline expectations generated by examination of stably expressed miRNAs, Gene Ontology (GO) analysis showed that these 629 DEGS were enriched for ion transport, pH regulation, calcium handling, endocytotic, and apoptotic activities. Seven differentially expressed miRNAs (miR-21, miR-31, miR-488, miR-153, miR-135b, miR-135a and miR298) in secretory- and/or maturation-stage enamel organs were confirmed by in situ hybridization. Further, we used luciferase reporter assays to provide evidence that two of these differentially expressed miRNAs, miR-153 and miR-31, are potential regulators for their predicated target mRNAs, Lamp1 (miR-153) and Tfrc (miR-31).

CONCLUSIONS

In conclusion, these data indicate that miRNAs exhibit a dynamic expression pattern during the transition from secretory-stage to maturation-stage tooth enamel formation. Although they represent only one of numerous mechanisms influencing gene activities, miRNAs specific to the maturation stage could be involved in regulating several key processes of enamel maturation by influencing mRNA stability and translation.

Adaptor protein complex 2-mediated, clathrin-dependent endocytosis, and related gene activities, are a prominent feature during maturation stage amelogenesis

Molecular events defining enamel matrix removal during amelogenesis are poorly understood. Early reports have suggested that adaptor proteins (AP) participate in ameloblast-mediated endocytosis. Enamel formation involves the secretory and maturation stages, with an increase in resorptive function during the latter. Here, using real-time PCR, we show that the expression of clathrin and adaptor protein subunits are upregulated in maturation stage rodent enamel organ cells. AP complex 2 (AP-2) is the most upregulated of the four distinct adaptor protein complexes. Immunolocalization confirms the presence of AP-2 and clathrin in ameloblasts, with strongest reactivity at the apical pole. These data suggest that the resorptive functions of enamel cells involve AP-2 mediated, clathrin-dependent endocytosis, thus implying the likelihood of specific membrane-bound receptor(s) of enamel matrix protein debris. The mRNA expression of other endocytosis-related gene products is also upregulated during maturation including: lysosomal-associated membrane protein 1 (Lamp1); cluster of differentiation 63 and 68 (Cd63 and Cd68); ATPase, H(+) transporting, lysosomal V0 subunit D2 (Atp6v0d2); ATPase, H(+) transporting, lysosomal V1 subunit B2 (Atp6v1b2); chloride channel, voltage-sensitive 7 (Clcn7); and cathepsin K (Ctsk). Immunohistologic data confirms the expression of a number of these proteins in maturation stage ameloblasts. The enamel of Cd63-null mice was also examined. Despite increased mRNA and protein expression in the enamel organ during maturation, the enamel of Cd63-null mice appeared normal. This may suggest inherent functional redundancies between Cd63 and related gene products, such as Lamp1 and Cd68. Ameloblast-like LS8 cells treated with the enamel matrix protein complex Emdogain showed upregulation of AP-2 and clathrin subunits, further supporting the existence of a membrane-bound receptor-regulated pathway for the endocytosis of enamel matrix proteins. These data together define an endocytotic pathway likely used by ameloblasts to remove the enamel matrix during enamel maturation.

Enamel matrix derivative: a review of cellular effects in vitro and a model of molecular arrangement and functioning

BACKGROUND

Enamel matrix derivative (EMD), the active component of Emdogain®, is a viable option in the treatment of periodontal disease owing to its ability to regenerate lost tissue. It is believed to mimic odontogenesis, though the details of its functioning remain the focus of current research.

OBJECTIVE

The aim of this article is to review all relevant literature reporting on the composition/characterization of EMD as well as the effects of EMD, and its components amelogenin and ameloblastin, on the behavior of various cell types in vitro. In this way, insight into the underlying mechanism of regeneration will be garnered and utilized to propose a model for the molecular arrangement and functioning of EMD.

METHODS

A review of in vitro studies of EMD, or components of EMD, was performed using key words "enamel matrix proteins" OR "EMD" OR "Emdogain" OR "amelogenin" OR "ameloblastin" OR "sheath proteins" AND "cells." Results of this analysis, together with current knowledge on the molecular composition of EMD and the structure and regulation of its components, are then used to present a model of EMD functioning.

RESULTS

Characterization of the molecular composition of EMD confirmed that amelogenin proteins, including their enzymatically cleaved and alternatively spliced fragments, dominate the protein complex (>90%). A small presence of ameloblastin has also been reported. Analysis of the effects of EMD indicated that gene expression, protein production, proliferation, and differentiation of various cell types are affected and often enhanced by EMD, particularly for periodontal ligament and osteoblastic cell types. EMD also stimulated angiogenesis. In contrast, EMD had a cytostatic effect on epithelial cells. Full-length amelogenin elicited similar effects to EMD, though to a lesser extent. Both the leucine-rich amelogenin peptide and the ameloblastin peptides demonstrated osteogenic effects. A model for molecular structure and functioning of EMD involving nanosphere formation, aggregation, and dissolution is presented.

CONCLUSIONS

EMD elicits a regenerative response in periodontal tissues that is only partly replicated by amelogenin or ameloblastin components. A synergistic effect among the various proteins and with the cells, as well as a temporal effect, may prove important aspects of the EMD response in vivo.

Amelogenins: Multi-Functional Enamel Matrix Proteins and Their Binding Partners

Amelogenins are the most abundant extracellular matrix proteins secreted by ameloblasts during tooth development and are important for enamel formation. Recently, amelogenins have been detected not only in ameloblasts, which are differentiated from the epithelial cell lineage, but also in other tissues, including mesenchymal tissues at low levels, suggesting that amelogenins possess other functions in these tissues. The therapeutic application of an enamel matrix derivative rich in amelogenins resulted in the regeneration of cementum, alveolar bone, and periodontal ligament (PDL) in the treatment of experimental or human periodontitis, indicating the attractive potential of amelogenin in hard tissue formation. In addition, a full-length amelogenin (M180) and leucine-rich amelogenin peptide (LRAP) regulate cementoblast/PDL cell proliferation and migration in vitro. Interestingly, amelogenin null mice show increased osteoclastogenesis and root resorption in periodontal tissues. Recombinant amelogenin proteins suppress osteoclastogenesis in vivo and in vitro, suggesting that amelogenin is involved in preventing idiopathic root resorption. Amelogenins are implicated in tissue-specific epithelial-mesenchymal or mesenchymal-mesenchymal signaling; however, the precise molecular mechanism has not been characterized. In this review, we first discuss the emerging evidence for the additional roles of M180 and LRAP as signaling molecules in mesenchymal cells. Next, we show the results of a yeast two-hybrid assay aimed at identifying protein-binding partners for LRAP. We believe that gaining further insights into the signaling pathway modulated by the multifunctional amelogenin proteins will lead to the development of new therapeutic approaches for treating dental diseases and disorders.

Structural analysis of a repetitive protein sequence motif in strepsirrhine primate amelogenin

Strepsirrhines are members of a primate suborder that has a distinctive set of features associated with the development of the dentition. Amelogenin (AMEL), the better known of the enamel matrix proteins, forms 90% of the secreted organic matrix during amelogenesis. Although AMEL has been sequenced in numerous mammalian lineages, the only reported strepsirrhine AMEL sequences are those of the ring-tailed lemur and galago, which contain a set of additional proline-rich tandem repeats absent in all other primates species analyzed to date, but present in some non-primate mammals. Here, we first determined that these repeats are present in AMEL from three additional lemur species and thus are likely to be widespread throughout this group. To evaluate the functional relevance of these repeats in strepsirrhines, we engineered a mutated murine amelogenin sequence containing a similar proline-rich sequence to that of Lemur catta. In the monomeric form, the MQP insertions had no influence on the secondary structure or refolding properties, whereas in the assembled form, the insertions increased the hydrodynamic radii. We speculate that increased AMEL nanosphere size may influence enamel formation in strepsirrhine primates.

Ameloblastin regulates osteogenic differentiation by inhibiting Src kinase via cross talk between integrin beta1 and CD63

Ameloblastin, the most abundant nonamelogenin enamel matrix protein, plays a role in ameloblast differentiation. Here, we found that ameloblastin was expressed in osteosarcoma cells; to explore the potential functions of ameloblastin in osteoblasts, we investigated whether this protein is involved in osteogenic differentiation and bone formation on the premise that CD63, a member of the transmembrane-4 glycoprotein superfamily, interacts with integrins in the presence of ameloblastin. Ameloblastin bound to CD63 and promoted CD63 binding to integrin β1. The interaction between CD63 and integrin β1 induced Src kinase inactivation via the binding of CD63 to Src. The reduction of Src activity and osteogenic differentiation mediated by ameloblastin were abrogated by treatment with anti-CD63 antibody and overexpression of constitutively active Src, respectively. Therefore, our results suggest that ameloblastin is expressed in osteoblasts and functions as a promoting factor for osteogenic differentiation via a novel pathway through the interaction between CD63 and integrin β1.

Excess fluoride interferes with chloride-channel-dependent endocytosis in ameloblasts

Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene cause cystic fibrosis (CF). Both CF and dental fluorosis result in protein retention in mature enamel. We hypothesized that excess fluoride might cause protein retention by interfering with CFTR function, resulting in abnormal expression of proteases and pathological endocytosis. Millimolar concentrations of fluoride reduced uptake of Emdogain, an enamel matrix derivative, in ameloblast-like PABSo-E cells, while stimulating an acidic intracellular environment at the same time. When CFTR function was inhibited by either an siRNA or a chloride channel inhibitor, CFTRinh-172, fluoride's effect on Emdogain uptake was partially blocked. Treatment of cells with CFTR siRNA down-regulated expression of proteases MMP20 and KLK4 and increased intracellular pH. We conclude that excess fluoride inhibits endocytic activity of ameloblasts through the CFTR chloride channel or other chloride channels. The intracellular pH might be the key mechanism by which abnormal proteolytic activity and defective endocytosis cause the residual protein observed in enamel of patients with CF and dental fluorosis.

Effects of human full-length amelogenin on the proliferation of human mesenchymal stem cells derived from bone marrow

Amelogenins are enamel matrix proteins that play a crucial role in enamel formation. Recent studies have revealed that amelogenins also have cell signaling properties. Although amelogenins had been described as specific products of ameloblasts, recent research has demonstrated their expression in bone marrow stromal cells. In this study, we examined the effect of recombinant human full-length amelogenin (rh174) on the proliferation of human mesenchymal stem cells (MSCs) derived from bone marrow and characterized the associated changes in intracellular signaling pathways. MSCs were treated with rh174 ranging in dose from 0 to 1,000 ng/ml. Cell proliferative activity was analyzed by bromodeoxyuridine (BrdU) immunoassay. The expression of lysosomal-associated membrane protein 1 (LAMP1), a possible amelogenin receptor, in MSCs was analyzed. Anti-LAMP1 antibody was used to block the binding of rh174 to LAMP1. The MAPK-ERK pathway was examined by Cellular Activation of Signaling ELISA (CASE) kit and western blot analysis. A specific MAPK inhibitor, U0126, was used to block ERK activity. It was shown that rh174 increased the proliferation of MSCs and MAPK-ERK activity. The MSC proliferation and MAPK-ERK activity enhanced by rh174 were reduced by the addition of anti-LAMP1 antibody. Additionally, the increased proliferation of MSCs induced by rh174 was inhibited in the presence of U0126. In conclusion, it is demonstrated that rh174 increases the proliferation of MSCs by interaction with LAMP1 through the MAPK-ERK signaling pathway, indicating the possibility of MSC application to tissue regeneration in the orofacial region.

Full length amelogenin binds to cell surface LAMP-1 on tooth root/periodontium associated cells

OBJECTIVES

Lysosome-associated membrane protein-1 (LAMP-1) has been suggested to be a cell surface receptor for a specific amelogenin isoform, leucine-rich amelogenin peptide or LRAP. However, it is unclear if LAMP-1 is an amelogenin receptor for dental mesenchymal cells. The goal of this study was to determine if LAMP-1 serves as a cell surface binding site for full length amelogenin on tooth root/periodontium associated mesenchymal cells.

DESIGN

Murine dental follicle cells and cementoblasts (OCCM-30) were cultured for 2 days followed by addition of full length recombinant mouse amelogenin, rp(H)M180. Dose-response (0-100 microg/ml) and time course (0-120 min) assays were performed to determine the optimal conditions for live cell surface binding using immunofluorescent microscopy. A competitive binding assay was performed to determine binding specificity by adding Emdogain (1 mg/ml) to the media. An antibody against LAMP-1 was used to detect the location of LAMP-1 on the cell surface and the pattern was compared to cell surface bound amelogenin. Both amelogenin and cell surface LAMP-1 were immuno-co-localized to compare the amount and distribution pattern.

RESULTS

Maximum surface binding was achieved with 50 microg/ml of rp(H)M180 for 120 min. This binding was specific as demonstrated by competitive inhibition (79% lower) with the addition of Emdogain. The binding pattern for rp(H)M180 was similar to the distribution of surface LAMP-1 on dental follicle cells and cementoblasts. The high co-localization coefficient (0.92) for rp(H)M180 and LAMP-1 supports rp(H)M180 binding to cell surface LAMP-1.

CONCLUSIONS

The data from this study suggest that LAMP-1 can serve as a cell surface binding site for amelogenin on dental follicle cells and cementoblasts.

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.

Amelogenin in cranio-facial development: the tooth as a model to study the role of amelogenin during embryogenesis

The amelogenins comprise 90% of the developing extracellular enamel matrix proteins and play a major role in the biomineralization and structural organization of enamel. Amelogenins were also detected, in smaller amounts, in postnatal calcifying mesenchymal tissues, and in several nonmineralizing tissues including brain. Low molecular mass amelogenin isoforms were suggested to have signaling activity; to produce ectopically chondrogenic and osteogenic-like tissue and to affect mouse tooth germ differentiation in vitro. Recently, some amelogenin isoforms were found to bind to the cell surface receptors; LAMP-1, LAMP-2 and CD63, and subsequently localize to the perinuclear region of the cell. The recombinant amelogenin protein (rHAM(+)) alone brought about regeneration of the tooth supporting tissues: cementum, periodontal ligament and alveolar bone, in the dog model, through recruitment of progenitor cells and mesenchymal stem cells. We show that amelogenin is expressed in various tissues of the developing mouse embryonic cranio-facial complex such as brain, eye, ganglia, peripheral nerve trunks, cartilage and bone, and is already expressed at E10.5 in the brain and eye, long before the initiation of tooth formation. Amelogenin protein expression was detected in the tooth germ (dental lamina) already at E13.5, much earlier than previously reported (E19). Application of amelogenin (rHAM(+)) beads together with DiI, on E13.5 and E14.5 embryonic mandibular mesenchyme and on embryonic tooth germ, revealed recruitment of mesenchymal cells. The present results indicate that amelogenin has an important role in many tissues of the cranio-facial complex during mouse embryonic development and differentiation, and might be a multifunctional protein.

Regeneration of bone and periodontal ligament induced by recombinant amelogenin after periodontitis

Regeneration of mineralized tissues affected by chronic diseases comprises a major scientific and clinical challenge. Periodontitis, one such prevalent disease, involves destruction of the tooth-supporting tissues, alveolar bone, periodontal-ligament and cementum, often leading to tooth loss. In 1997, it became clear that, in addition to their function in enamel formation, the hydrophobic ectodermal enamel matrix proteins (EMPs) play a role in the regeneration of these periodontal tissues. The epithelial EMPs are a heterogeneous mixture of polypeptides encoded by several genes. It was not clear, however, which of these many EMPs induces the regeneration and what mechanisms are involved. Here we show that a single recombinant human amelogenin protein (rHAM⁺), induced in vivo regeneration of all tooth-supporting tissues after creation of experimental periodontitis in a dog model. To further understand the regeneration process, amelogenin expression was detected in normal and regenerating cells of the alveolar bone (osteocytes, osteoblasts and osteoclasts), periodontal ligament, cementum and in bone marrow stromal cells. Amelogenin expression was highest in areas of high bone turnover and activity. Further studies showed that during the first 2 weeks after application, rHAM⁺ induced, directly or indirectly, significant recruitment of mesenchymal progenitor cells, which later differentiated to form the regenerated periodontal tissues. The ability of a single protein to bring about regeneration of all periodontal tissues, in the correct spatio-temporal order, through recruitment of mesenchymal progenitor cells, could pave the way for development of new therapeutic devices for treatment of periodontal, bone and ligament diseases based on rHAM⁺.

The effects of LAMP1 and LAMP3 on M180 amelogenin uptake, localization and amelogenin mRNA induction by amelogenin protein

We previously demonstrated that the uptake of M180 amelogenin protein in dental epithelial cells (HAT-7) results in increased levels of amelogenin mRNA through enhanced mRNA stabilization. To determine the processes involved in the uptake of extracellular M180 amelogenin by cells and in amelogenin intracellular trafficking in the amelogenin protein-mediated amelogenin mRNA expression pathway, we investigated the effects of LAMP1 and LAMP3, which are candidate M180 amelogenin receptors, on M180 amelogenin uptake, localization and amelogenin mRNA induction by amelogenin protein, using anti-LAMP-1 and anti-LAMP-3 antibodies and siRNA analysis. The results indicate that LAMP3 blocking by anti-LAMP-3 decreases M180 amelogenin uptake, but does not affect amelogenin mRNA induction by amelogenin protein, suggesting that LAMP3 is related to amelogenin degradation. Down-regulation by siRNA of LAMP1, which is the receptor for small amelogenin protein (LRAP), does not affect M180 amelogenin uptake, localization or amelogenin mRNA induction by amelogenin protein. Thus, while LAMP1 is the specific receptor for LRAP, it is not a receptor for M180 amelogenin. These findings will aid further research into the understanding of M180 amelogenin function and expression.

The cell adhesion molecule nectin-1 is critical for normal enamel formation in mice

Nectin-1 is a member of a sub-family of immunoglobulin-like adhesion molecules and a component of adherens junctions. In the current study, we have shown that mice lacking nectin-1 exhibit defective enamel formation in their incisor teeth. Although the incisors of nectin-1-null mice were hypomineralized, the protein composition of the enamel matrix was unaltered. While strong immunostaining for nectin-1 was observed at the interface between the maturation-stage ameloblasts and the underlying cells of the stratum intermedium (SI), its absence in nectin-1-null mice correlated with separation of the cell layers at this interface. Numerous, large desmosomes were present at this interface in wild-type mice; however, where adhesion persisted in the mutant mice, the desmosomes were smaller and less numerous. Nectins have been shown to regulate tight junction formation; however, this is the first report showing that they may also participate in the regulation of desmosome assembly. Importantly, our results show that integrity of the SI–ameloblast interface is essential for normal enamel mineralization.