The Function of Leucine-Rich Amelogenin Peptide in tooth formation

Background:  LRAP, or leucine rich amelogenin peptide (LRAP) is a 59 amino acid alternatively spliced amelogenin, which includes the first 33 and the last 26 amino acids from the full-length amelogenin. LRAP has been shown to have cell signaling properties, though its function in enamel formation remains unknown.

Objective: To determine the effects of LRAP in mouse tooth formation, using a transgenic mouse, which overexpresses LRAP.

Method: Hemi-maxillas of wild-type (WT), and LRAP overexpressor in wild-type background (LRAP/WT) mice (0 and 5 days post-natal) were fixed in 4% paraformaldehyde, and demineralized in 8% EDTA (Ph 7.6) AT 4˚C.  The jaws were   embedded in paraffin for longitudinal sectioning.  The localization of various matrix, differentiation, and signaling proteins in the developing tooth organ were compared by immunohistochemistry (IHC). Immunoreactivity was detected with a Vecastatin ABC kit. Right dry hemimandibles were scanned with a microCT to characterize the mineral density of the dental tissues in WT and transgenic mice.

Result: In both newborn and day 5 mice, ameloblast morphology was disrupted, suggesting an interference with cell polarization.  IHC In newborn LRAP/WT mice  revealed a transient upregulation of amelogenin in secretory ameloblasts and adjacent odontoblasts, with no similar upregulation in WT mouse molars. In the maxillary first molar of postnatal day 5 LRAP/WT mice, ameloblasts were poorly organized and enamel matrix formation was almost completely absent.  Amelogenin immunostaining in the cell cytoplasm of LRAP/WT mice was increased as compared to controls, and Lamp1 was upregulated in both ameloblasts and odontoblasts of DPN5 LRAP/WT mice, Initial microCT data showed an overall delay in mandibular incisor and molar development in the LRAP/WT mice at the early post-natal stage.

Conclusion: Early overexpression of LRAP interferes with ameloblast function and tooth development.