Posteruptive Loss of Proteins in Porcine Enamel

Abstract

Tooth enamel maturation requires the removal of proteins from the mineralizing enamel matrix to allow for crystallite growth until full hardness is reached to meet the mechanical needs of mastication. While this process takes up to several years in humans before the tooth erupts, it is greatly accelerated in the faster-developing pigs. Pig teeth erupt with softer, protein-rich enamel that is similar to hypomineralized human enamel but continues to harden quickly after eruption. Proteins that bind to enamel crystals and prevent crystal growth and enamel hardening (e.g., albumin) have been suggested as a cause for hypomineralized human enamel. It is unclear whether fast posteruptive enamel hardening in pigs occurs despite the high protein content or requires facilitated protein loss for crystal growth. This study asked how the protein content in porcine enamel changes after eruption in relation to saliva. Given previous evidence of high albumin content in erupted porcine enamel, we hypothesized that enamel- and saliva-derived enzymes facilitate protein removal from porcine enamel after eruption. To test this, we analyzed the enamel of fourth primary premolars and the saliva proteome at 3 critical time points: at the time of tooth eruption and 2 and 6 weeks after eruption. We found a decrease in the number of proteins and their abundancy in enamel with posteruptive time, including a decrease in serum albumin within enamel. The rapid decrease within 2 weeks posteruption is consistent with the previously reported rapid increase in mineral density of porcine enamel after eruption. In addition to enamel proteases KLK-4 and MMP-20, we identified other serine-, cysteine-, aspartic-, and metalloproteases in enamel that are found in the porcine saliva. Our findings suggest that the fast posteruptive enamel maturation in the porcine model coincides with saliva exchange and influx of saliva enzymes into porous enamel.