IJPCDR An10.5005/jp-journals-10052-0018 Overview of Enamel Matrix Proteins REVIEW ARTICLE An Overview of Enamel Matrix Proteins 1Chaitradevi Saxena, 2Kartikay Saxena, 3Vikas Bhakhar, 4M Vidya, 5Mohsin Ghanchi, 6Dhaval Jani ABSTRACT non-collagenous proteins only and contains several Enamel, one of the hardest tissues of the body, consists of enamel proteins and enzymes. The major part of these 96% inorganic and 4% organic content. The inorganic content enamel proteins is composed of a heterogeneous group is made up of hydroxyapatite crystals, while the organic of low-molecular-weight proteins known as amelo- content is composed of non-collagenous proteins, the major genins, which may constitute as much as 90% of the part of which is enamel proteins. The understanding of enamel total. The remaining 10% consists of non-amelogenins. proteins has undergone a sea change in the last few years. Enamelin and ameloblastin are the two most important Primarily, enamel proteins are divided into amelogenin and 2,3 non-amelogenin families. Amelogenins are believed to regulate non-amelogenins. the growth, thickness, and width of hydroxyapatite crystals. The The extracellular matrix of the enamel is now reason- expression of amelogenins has been shown to have a role in ably well defined in terms of its major protein compo- sex determination as well. Non-amelogenins include other pro- nents. Unlike other mineralized tissues, a forming enamel teins like ameloblastin, enamelin, enamelysin, and APin. They does not exhibit a distinct unmineralized matrix like are shown to be involved in cell matrix interactions, periodontal osteoid or predentin. The hydroxyapatite crystals grow regeneration, and proper enamel development. Recent studies have also shown the role of enamel proteins in understanding directly against the secretory surfaces of the ameloblasts. the nature of odontogenic tumors. Although the background matrix formed by the margin- ally soluble amelogenins may provide some physical Keywords: Ameloblastin, Amelogenin, Enamel, Enamelin, Enamelysin, Protein support, enamel proteins likely do not play any structur- ing function.1 Morphologically, the organic matrix of a How to cite this article: Saxena C, Saxena K, Bhakhar V, Vidya M, Ghanchi M, Jani D. An Overview of Enamel Matrix Proteins. Int forming enamel appears uniform in decalcified histologic J Prev Clin Dent Res 2016;3(1):79-84. preparations. However, immunohistochemical studies reveal that enamel proteins are differentially distributed Source of support: Nil across the enamel layer. It was found that intact or rela- Conflict of interest: None tively intact non-amelogenin molecules are concentrated near the cell surface, whereas mostly degraded fragments INTRODUCTION are found in the deeper enamel.4 Enamel has evolved as an epithelially derived protective The exact nature and function of enamel proteins is covering for teeth. Being one of the hardest tissues of the a subject that is still under extensive research. Multiple body, it along with dentin serves to protect the underlying studies have been conducted to elucidate the exact pulp. Fully formed enamel is the most highly mineralized effects of these proteins on the development of the tooth extracellular matrix known, consisting of approximately structure. In spite of this, many questions about the 96% mineral and 4% organic material and water.1 enamel proteins are still unanswered. The inorganic content of enamel is crystalline calcium phosphate (hydroxyapatite) substituted with carbonate MELOGENIN FAMILY ions. The organic matrix of the enamel is made from This consists exclusively of the protein amelogenin, which forms the bulk of the total enamel proteins. 1,2,6Reader, 3Senior Lecturer, 4,5Professor and Head Amelogenins 1Department of Conservative Dentistry, College of Dental Sciences, Bhavnagar, Gujarat, India Heterogeneous amelogenins are hydrophobic proteins rich 2,3,5,6Department of Oral Pathology, College of Dental Sciences in proline, histidine, and glutamine. Their heterogeneity Bhavnagar, Gujarat, India is brought about in three ways: 4Department of Oral Pathology, Yenapoya Dental College • The genes responsible for transcribing amelogenin are Mangaluru, Karnataka, India found on X and Y chromosomes, and because these Corresponding Author: Vikas Bhakhar, Senior Lecturer 2 genes are not 100% homologous, a sexual heteroge- Department of Oral Pathology, College of Dental Sciences neity exists from the outset.5 Amargadh, Bhavnagar, Gujarat, India, Phone: +919638983058 • The amelogenin gene contains at least 7 exons, which e-mail: [email protected] can be spliced in numerous ways to produce mature International Journal of Preventive and Clinical Dental Research, January-March 2016;3(1):79-84 79 Chaitradevi Saxena et al messenger ribonucleic acids that may include all (amplicon), and the Y chromosome gene, AMELY, a 112 bp 7 exons or a lack of some of them.6 amplicon. Hence, the AMELX contains a 6 bp deletion in • Amelogenins undergo short-term and long-term the intron 1. Therefore, when the amplicons are run on an extracellular processing by proteolytic enzymes into agarose gel, samples from male sources (XY) will show lower-molecular-weight fragments. Of these, the two bands on an agarose gel (one for the 106 bp fragment tyrosine-rich amelogenin polypeptide (TRAP) and and another for the 112 bp fragment), while females (XX) leucine-rich amelogenin polypeptide (LRAP) are sig- will show only one band. Thus, this process allows for nificant because they constitute the bulk of the final sex determination of unknown samples.9,14 organic matrix of a maturing enamel.7,8 However, mutations in the Y-derived fragment of the gene may result in amplification failure of the Y allele, Applications of Amelogenins causing misidentification of the biological sample as of a female. The error rate is not much. Indians, however, seem Although not completely understood, the function of to have an unusually high rate of amelogenin deletion in amelogenins is believed to be in organizing enamel rods Y chromosomes. In one study, Thangaraj et al3 studied a during tooth development. The latest research indicates total of 270 male samples, of which 5 showed a deletion that this protein regulates the initiation and growth of of Y chromosome–specific amelogenin (1.85%). hydroxyapatite crystals during the mineralization of Amelogenin has also been shown to be a cell adhesion enamel.9 They are supposed to regulate the growth in protein. It has been shown that an amelogenin-containing thickness and width of the crystals. Other studies show preparation, emdogain, possesses cell-adhesive activity. that the amelogenin has an inhibitory effect on the crystal Studies showed that amelogenin and emdogain can growth, and this is achieved by the selective adsorp- promote the adhesion of many cell types via a divalent tion of 25 kDa amelogenin on the surface of the apatite cation-dependent mechanism. It is known that amelogenin crystals. The interaction of amelogenins with enamel binds to minerals; studies have shown that amelogenin crystals may affect the progress of enamel mineralization does not bind to collagen or heparin under physiological in several ways: conditions. Amelogenin shows characteristics similar to • The habit of enamel crystals could be determined in the surface adherent material (SAM) class of cell adhesion part by the preferential adsorption of the molecules proteins. It is not membrane intercalated and requires onto either the basal or the prism face, yielding different divalent cations for activity. While the SAMs often have growth kinetics between these crystallographic a binding site for collagen, amelogenin apparently does planes.10 not. In contrast, amelogenin does bind to hydroxyapatite • The apatitic surfaces might cause an accelerated and not only supports cell adhesion but also promotes cell degradation of the adsorbed molecules or indirectly spreading.15 The results of various studies showed that control the enzymatic degradation of amelogenins; since amelogenin binds to hydroxyapatite and is present most likely the conformation of amelogenins on the in the organic matrix of developing teeth, amelogenin crystal surfaces is different from those of the molecules may mediate the adhesion of ameloblasts and other cell 11 in liquid phase. types to the mineral component of a developing teeth.16 • The removal of degraded products of amelogenins Apart from their roles related to enamel in particular, might be retarded through their adsorption onto amelogenins have been shown to have an effect on the forming or formed crystals, especially if the other dental tissues as well.7 As gene expression assays adsorption affinities of amelogenin moieties are became more sensitive, expression was also noted in enhanced by changes in the nature of the crystal tissues not involved with enamel formation leading to 12 surfaces during successive developmental stages. hypotheses concerning additional roles for these proteins. Another use of amelogenin is in sex determination. As In vitro approaches led to the discovery that some of the mentioned earlier, the genes responsible for transcribing amelogenins are able to regulate gene expression and to amelogenins are found on X and Y chromosomes.5 This participate in cellular signaling. An extract containing heterogeneity is used to help in sex determination. For predominately amelogenins has been used clinically in the sex determination using amelogenin, the technique the treatment of certain forms of