Abstract
Amelotin (AMTN) is an ameloblast-secreted protein that belongs to the secretory calcium-binding phosphoprotein family, which also includes the enamel matrix proteins amelogenin, ameloblastin and enamelin. Although AMTN is supposed to play an important role in enamel formation, data were long limited to the rodents, in which it is expressed during the maturation stage. Recent comparative studies in sauropsids and amphibians revealed that (i) AMTN was expressed earlier, i.e. as soon as ameloblasts are depositing the enamel matrix, and (ii) AMTN structure was different, a change which mostly resulted from an intraexonic splicing in the large exon 8 of an ancestral mammal. The present study was performed to know whether the differences in AMTN structure and expression in rodents compared to non-mammalian tetrapods dated back to an early ancestral mammal or were acquired later in mammalian evolution. We sequenced, assembled and screened the jaw transcriptome of a neonate opossum Monodelphis domestica, a marsupial. We found two AMTN transcripts. Variant 1, representing 70.8% of AMTN transcripts, displayed the structure known in rodents, whereas variant 2 (29.2%) exhibited the nonmammalian tetrapod structure. Then, we studied AMTN expression during amelogenesis in a neonate specimen. We obtained similar data as those reported in rodents. These findings indicate that more than 180 million years ago, before the divergence of marsupials and placentals, changes occurred in AMTN function and structure. The spatiotemporal expression was delayed to the maturation stage of amelogenesis and the intraexonic splicing gave rise to isoform 1, encoded by variant 1 and lacking the RGD motif. The ancestral isoform 2, housing the RGD, was initially conserved, as demonstrated here in a marsupial, then secondarily lost in the placental lineages. These findings bring new elements towards our understanding of the non-prismatic to prismatic enamel transition that occurred at the onset of mammals.
Highlights
Amelotin (AMTN) belongs to the secretory calcium-binding phosphoprotein (SCPP) family, and more precisely to the Pro-Glu rich sub-family, which includes the three enamel matrix proteins (EMPs), amelogenin (AMEL), ameloblastin (AMBN) and enamelin (ENAM) [1]
AMTN is expressed by ameloblasts from maturation stage onwards [3] while in lizard and salamander, AMTN transcripts were detected from early enamel matrix formation to late enamel maturation [4]
Alignment with the putative gDNA sequence revealed that these transcripts corresponded to two AMTN variants, variant 1 (V1) and variant 2 (V2) respectively, and included the 3' and 5' UTR
Summary
Amelotin (AMTN) belongs to the secretory calcium-binding phosphoprotein (SCPP) family, and more precisely to the Pro-Glu rich sub-family, which includes the three enamel matrix proteins (EMPs), amelogenin (AMEL), ameloblastin (AMBN) and enamelin (ENAM) [1]. AMTN is expressed by ameloblasts from maturation stage onwards [3] while in lizard and salamander, AMTN transcripts were detected from early enamel matrix formation to late enamel maturation [4]. These findings suggest different functions of AMTN during amelogenesis in non-mammalian tetrapods compared to rodents. In the latter it was suggested that AMTN could play a role in the adherence of ameloblasts and of the junctional epithelium to the enamel surface, or/and in the mineralization of the outer aprismatic enamel layer [5,6,7,8]. The AMTN function(s) as proposed in rodents could date back to this ancient geological period (310–200 Ma), during which early mammals acquired dental occlusion and prismatic enamel [10]
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