Function of Amelogenins in Porcine Enamel Mineralization During the Secretory Stage of Amelogenesis

Abstract

The purpose of this investigation was to study the properties of amelogenins present in the secretory stage of amelogenesis of porcine enamel, and to explore their possible role in enamel formation. Interest was focused on three species of amelogenins having different properties regarding localization in the forming enamel, solubility, adsorption affinity onto hydroxyapatite (HA), and inhibitory activity of HA growth. The results suggest that the secreted 25-kilo-Dalton (kDa) parent amelogenin may directly regulate the growth kinetics and the habit of forming crystals through selective adsorption onto specific crystal surfaces. The observed constant distribution of 20-kDa amelogenin during the secretory stage of amelogenesis may be explained by steady-state conditions of two degradation processes: from the 25-kDa to the 20-kDa species; and, subsequently, from the 20-kDa to the 13-kDa and 11-kDa species. The former process could be crucial to induction of a slow growth of the crystals in directions of width and thickness, as observed following the initial, preferential growth parallel to the long-axis direction. Possible modulation of this slow growth by the presence of 20-kDa amelogenin as a weak inhibitor or a space-filling matrix through the secretory stage is still a very open question. The concentration gradient of the soluble amelogenins from the inner to the outer enamel would promote the diffusion toward the ameloblasts and the removal of the amelogenins by the cells. It should be pointed out that the soluble moieties (13 kDa and 11 kDa) in the fluid may also play a regulatory function with respect to the driving force for crystal growth through a weak Ca-binding, thus reducing the ionic calcium activity.