Amelotin: an enamel matrix protein that experienced distinct evolutionary histories in amphibians, sauropsids and mammals

Abstract

BackgroundAmelotin (AMTN) is an ameloblast-secreted protein that belongs to the secretory calcium-binding phosphoprotein (SCPP) family, which originated in early vertebrates. In rodents, AMTN is expressed during the maturation stage of amelogenesis only. This expression pattern strongly differs from the spatiotemporal expression of other ameloblast-secreted SCPPs, such as the enamel matrix proteins (EMPs). Furthermore, AMTN was characterized in rodents only. In this study, we applied various approaches, including in silico screening of databases, PCRs and transcriptome sequencing to characterize AMTN sequences in sauropsids and amphibians, and compared them to available mammalian and coelacanth sequences.ResultsWe showed that (i) AMTN is tooth (enamel) specific and underwent pseudogenization in toothless turtles and birds, and (ii) the AMTN structure changed during tetrapod evolution. To infer AMTN function, we studied spatiotemporal expression of AMTN during amelogenesis in a salamander and a lizard, and compared the results with available expression data from mouse. We found that AMTN is expressed throughout amelogenesis in non-mammalian tetrapods, in contrast to its expression limited to enamel maturation in rodents.ConclusionsTaken together our findings suggest that AMTN was primarily an EMP. Its functions were conserved in amphibians and sauropsids while a change occurred early in the mammalian lineage, modifying its expression pattern during amelogenesis and its gene structure. These changes likely led to a partial loss of AMTN function and could have a link with the emergence of prismatic enamel in mammals.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0329-x) contains supplementary material, which is available to authorized users.

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

“…Tomes’ processes and the prismatic organization of enamel crystallites arose relatively recently during evolution, in mammals before the divergence of marsupials and placental mammals (Gasse et al. ), about 160 Ma (Luo et al. ).…”

Maturation stage enamel malformations in Amtn and Klk4 null mice

“…During evolution, important structural and regulatory changes have occurred in the Amtn gene [13,9]. Exon 2b encoding a Golgi casein kinase phosphorylation site and the 3’ part of Exon 8 encoding an RGD integrin binding site were lost, and expression of Amtn , previously initiating early during the secretory stage, was delayed to the transition to maturation [13,14].…”

“…Exon 2b encoding a Golgi casein kinase phosphorylation site and the 3’ part of Exon 8 encoding an RGD integrin binding site were lost, and expression of Amtn , previously initiating early during the secretory stage, was delayed to the transition to maturation [13,14]. These changes are associated with the emergence of prismatic enamel in mammals, and suggest that AMTN function became modified in stem mammals.…”

“…The SCPP basal lamina protein SCPPPQ1 that is expressed late in the maturation stage [10,11] arose in reptiles [12]. The organization of thin crystal ribbons into rod and interrod structures is an innovation that occurred in mammals before the divergence of marsupials and may be associated with evolutionary changes in amelotin structure and a delay in its expression until completion of the secretory stage [13,14,12]. The secretion of kallikrein 4 (KLK4) to facilitate the maturation of dental enamel was a late addition evolutionarily, which occurred in placental mammals and is associated with earlier eruption of the permanent (secondary) dentition [15].…”

Deletion of amelotin exons 3–6 is associated with amelogenesis imperfecta

“…It has been proposed that prismatic enamel evolved in mammals concurrently with AMTN expression becoming restricted to the maturation stage and with changes to AMTN exon boundaries and splicing (34,45,46). The exact function of AMTN in mammals remains unknown, but its expression within the basal lamina of maturation stage ameloblasts and at the internal basal lamina within the junctional epithelium of erupted rodent teeth suggests functions in both enamel mineralization and in dento-gingival attachment (26,27).…”