Comparative HPLC, SDS-PAGE, and immunoblot analyses of dental enamel proteins

Identification and characterization of the cDNA sequence encoding amelogenin in rabbit (Oryctolagus cuniculus)

Amelogenins, the most abundant proteins in tooth enamel extracellular matrix (ECM), are essential for tooth amelogenesis. The nucleotide sequence of amelogenin gene (AMEL) for rabbit, as an important member of mammals and good continuously growing incisor model, is important for comparative and evolutional study. Previous studies about rabbit amelogenin proteins got no consensus yet even as to their existence or size. In this study, with combined usage of in silico and molecular cloning technologies, we identified sequences of two transcripts of rabbit amelogenin, resulting from the alternative splicing of the 45-bp exon 4. The coding regions of the two transcripts are of 567- and 522-bp, encoding 188 and 173 amino acids including a 17-residue signal peptide, respectively. Sequence analysis revealed that rabbit amelogenin features in extremely high GC-content in nucleotide sequence and Alanine content in protein sequence. Detailed comparison of amino acid sequence with other mammals showed that the rabbit amelogenin protein is conserved in the sites and regions important for protein functions. Overall, our results uncovered the mysteries about rabbit amelogenin and revealed its sequence peculiarities.

Slc26a3/Dra and Slc26a6 in Murine Ameloblasts

Formation of apatite crystals during enamel development generates protons. To sustain mineral accretion, maturation ameloblasts need to buffer these protons. The presence of cytosolic carbonic anhydrases, the basolateral Na(+) bicarbonate cotransporter Nbce1, and the basolateral anion exchanger Ae2a,b in maturation ameloblasts suggests that these cells secrete bicarbonates into the forming enamel, but it is unknown by which mechanism. Solute carrier (Slc) family 26A encodes different anion exchangers that exchange Cl(-)/HCO3 (-), including Slc26a3/Dra, Slc26a6/Pat-1, and Slc26a4/pendrin. Previously, we showed that pendrin is expressed in ameloblasts but is not critical for enamel formation. In this study, we tested the hypothesis that maturation ameloblasts express Dra and Slc26a6 to secrete bicarbonate into the enamel space in exchange for Cl(-). Real-time polymerase chain reaction detected mRNA transcripts for Dra and Slc26a6 in mouse incisor enamel organs, and Western blotting confirmed their translation into protein. Both isoforms were immunolocalized in ameloblasts, principally at maturation stage. Mice with null mutation of either Dra or Slc26a6 had a normal dental or skeletal phenotype without changes in mineral density, as measured by micro-computed tomography. In enamel organs of Slc26a6-null mice, Dra and pendrin protein levels were both elevated by 52% and 55%, respectively. The amount of Slc26a6 protein was unchanged in enamel organs of Ae2a,b- and Cftr-null mice but reduced in Dra-null mice by 36%. Our data show that ameloblasts express Dra, pendrin, or Slc26a6 but each of these separately is not critical for formation of dental enamel. The data suggest that in ameloblasts, Slc26a isoforms can functionally compensate for one another.

Purification and analysis of a 5kDa component of enamel matrix derivative

High performance liquid chromatography (HPLC) methods were used to analyse a 5kDa component purified from enamel matrix derivative (EMD), the active ingredient in Emdogain, a commercial product for periodontal tissue regeneration. After initial purification by size-exclusion chromatography (SEC) on a 100 cm x 5 cm column (Bio-Gel P-30 Fine, 280 nm), collected fractions were analysed by size-exclusion HPLC (SE HPLC; TSK-Gel Super SW2000, 220 nm). The fractions containing only the 5kDa component were analysed by reversed-phase high-pressure chromatography (RP HPLC; YMC-Pack ODS-A, 200 nm), revealing four peaks of the 5kDa component. From 1200 mg of EMD (of which 9% is the 5kDa component), approximately 65 mg of lyophilised 5kDa component were obtained, corresponding to a recovery of 60%. The SE HPLC method was mainly suitable for qualitative analysis, whereas the RP HPLC method was appropriate for both qualitative and quantitative analysis.

Enamelysin (matrix metalloproteinase 20)-deficient mice display an amelogenesis imperfecta phenotype

Enamelysin is a tooth-specific matrix metalloproteinase that is expressed during the early through middle stages of enamel development. The enamel matrix proteins amelogenin, ameloblastin, and enamelin are also expressed during this same approximate developmental time period, suggesting that enamelysin may play a role in their hydrolysis. In support of this interpretation, recombinant enamelysin was previously demonstrated to cleave recombinant amelogenin at virtually all of the precise sites known to occur in vivo. Thus, enamelysin is likely an important amelogenin-processing enzyme. To characterize the in vivo biological role of enamelysin during tooth development, we generated an enamelysin-deficient mouse by gene targeting. Although mice heterozygous for the mutation have no apparent phenotype, the enamelysin null mouse has a severe and profound tooth phenotype. Specifically, the null mouse does not process amelogenin properly, possesses an altered enamel matrix and rod pattern, has hypoplastic enamel that delaminates from the dentin, and has a deteriorating enamel organ morphology as development progresses. Our findings demonstrate that enamelysin activity is essential for proper enamel development.

Biochemical characterization of recombinant mouse amelogenins: protein quantitation, proton absorption, and relative affinity for enamel crystals

Four recombinant mouse amelogenins, which varied by the presence or absence of the exon 4 encoded segment as well as the carboxyl-terminus were heterologously expressed and purified from bacteria. The rM193 and rM179 contain the carboxyl-terminus, whereas the rM180 and rM166 do not. The rM193 and rM180 contain the polypeptide segment encoded by exon 4 of the amelogenin gene. A precisely weighed sample of purified rM179 was quantified by Lowry, Bicinchoninic Acid and Bradford assays. It was determined that these protein quantification methods characteristically under or overestimate the amount of amelogenin. The calculated correction factors were: Lowry (x 1.35), BCA (x 1.96), and Bradford (x 0.78). Recombinant mouse amelogenin (rM179) was characterized with respect to its hydrogen ion binding properties. The protein absorbs 11.9 +/- 1.7 protons during a pH change from 8.0 to 5.0, suggesting that amelogenins buffer the enamel fluid in vivo. Crystal binding experiments were performed using rM193, rM180, rM179 and rM166. The carboxyl-terminus enhanced the binding of amelogenin to enamel crystals while the exon 4 encoded segment did not appreciably affect crystal binding.

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.

Characterization of recombinant pig enamelysin activity and cleavage of recombinant pig and mouse amelogenins

Enamelysin (MMP-20) is a tooth-specific matrix metalloproteinase that is initially expressed by ameloblasts and odontoblasts immediately prior to the onset of dentin mineralization, and continues to be expressed throughout the secretory stage of amelogenesis. During the secretory stage, enamel proteins are secreted and rapidly cleaved into a large number of relatively stable cleavage products. Multiple proteinases are present in the developing enamel matrix, and the precise role of enamelysin in the processing of enamel proteins is unknown. We have expressed, activated, and purified the catalytic domain of recombinant pig enamelysin, and expressed a recombinant form of the major secreted pig amelogenin rP172. These proteins were incubated together, and the digestion products were analyzed by SDS-PAGE and mass spectrometric analyses. We assigned amelogenin cleavage products by selecting among the possible polypeptides having a mass within 2 Daltons of the measured values. The polypeptides identified included the intact protein (amino acids 2-173), as well as 2-148, 2-136, 2-107, 2-105, 2-63, 2-45, 46-148, 46-147, 46-107, 46-105, 64-148, 64-147, and 64-136. These fragments of rP172 include virtually all of the major amelogenin cleavage products observed in vivo. We propose that enamelysin is the predominant proteinase that processes enamel proteins during the secretory phase of amelogenesis.

Cloning, characterization, and heterologous expression of exon-4-containing amelogenin mRNAs

The formation of dental enamel is dependent upon amelogenins, a family of proteins constituting most of the developing enamel matrix. Depending upon the species, these enamel proteins are expressed from either one or two copies of the amelogenin gene. Each gene directs the synthesis of a variety of amelogenin isoforms through alternative splicing of their pre-mRNA transcript(s). Before the role of amelogenins in dental enamel formation can be better understood, one must know the isoforms that are secreted and their biochemical properties. Previously, we cloned and characterized 7 mouse amelogenin RNA messages generated by alternative splicing. The largest amelogenin cDNA encoded a 194-residue amelogenin isoform which was the only clone to contain the 42-nucleotide exon 4 segment. Anti-peptide antibodies raised against the derived translation of this exon revealed an unexpectedly diverse assortment of murine amelogenins, suggesting that additional splicing variants could contain the exon 4 coding region. Using exon-4-specific oligonucleotide primers, we have amplified, cloned, and characterized three different amelogenin RNA messages. These messages encode amelogenin polypeptides (exclusive of signal peptides) 194, 170, and 73 amino acids in length. The isotope-averaged molecular weights for the deduced, single-phosphorylated, proteins are 21,897.1, 19,113.9, and 8176.5 Daltons, respectively. Splice-site selection for the generation of these mRNAs was identical to that of the previously characterized messages for the M180, M156, and M59 except for the inclusion of exon 4. The exon-4-containing amelogenin isoforms were heterologously expressed in E. coli by means of the pET11 expression system (Novagen, Madison, WI).