M180 amelogenin processed by MMP20 is sufficient for decussating murine enamel

Area of Science:

• Biochemistry
• Developmental Biology
• Materials Science

Background:

• Amelogenin (AMELX) and matrix metalloproteinase-20 (MMP20) are critical proteins for tooth enamel formation.
• Mutations in AMELX or MMP20 lead to amelogenesis imperfecta, a developmental disorder affecting enamel quality.
• MMP20 processes AMELX, a major enamel protein, during the secretory stage of enamel development.

Purpose of the Study:

• To investigate the specific role of MMP20 in processing the abundant M180 amelogenin isoform.
• To determine the sufficiency of the M180 amelogenin isoform for normal enamel structure and mechanical properties.

Main Methods:

• Generation of double knockout (DKO) mice lacking both Amelx and Mmp20, with a transgene expressing the M180 amelogenin isoform.
• Analysis of enamel phenotype using scanning electron microscopy (SEM) for structure, micro-computed tomography (µCT) for thickness, and nanoindentation for mechanical properties.

Main Results:

• Enamel from M180 transgenic/DKO mice showed significantly reduced hardness (37% of control) and lacked normal prismatic architecture.
• While thinner than wild-type, enamel from M180 transgenic/Amelx knockout mice retained normal mechanical properties and prismatic patterns.
• Loss of MMP20 in M180 transgenic/DKO mice abolished the normal prismatic structure observed in M180 transgenic/Amelx knockout mice.

Conclusions:

• The M180 amelogenin isoform alone is sufficient to establish normal enamel mechanical properties and prismatic patterns.
• Additional amelogenin splice products, beyond M180, are necessary for achieving normal enamel thickness.
• MMP20's processing of amelogenins, including M180, is essential for normal enamel structural integrity.