XBP1 Expression May Determine the Size of the Ameloblast ER

Ameloblasts progress through defined stages of development as enamel forms on teeth. Pre-secretory ameloblasts give rise to tall columnar secretory ameloblasts that direct the enamel to achieve its full thickness. During the maturation stage, the ameloblasts shorten and direct the enamel to achieve its final hardened form. Once the enamel is fully mature, ameloblasts regress and become part of the reduced enamel organ that covers and protects the completed enamel surface until the tooth erupts. Objective: Here we quantify the volume of selected ameloblast organelles (percent volume per ameloblast) as ameloblasts progress through six defined developmental stages. Because the volume of the ameloblast endoplasmic reticulum (ER) changed dramatically as a function of developmental stage, we also asked if X-box-binding protein-1 (XBP1) plays a role in regulating ameloblast ER volume as has been previously demonstrated for secretory ancinar cells and for plasma cell differentiation. Methods: Stereology was used to quantify the volume of ameloblast organelles, such as mitochondria, over the course of six stages of enamel development (pre-secretory, early secretory, late secretory, early maturation, late maturation, and regression). Immunohistochemistry identified the cells within the enamel organ that activate and express XBP1. Expression of the active form of XBP1 was detected by restriction digestion of the active splice form cDNA and by qPCR of enamel organ cDNA. Results: We demonstrate that mitochondria volume peaks during late maturation indicating that maturation stage ameloblasts maintain a high level of metabolic activity. Also, the ER volume changes significantly as a function of developmental stage and the level of Xbp1 expression correlates positively with percent volume of ameloblast ER. Conclusions: We conclude that the level of active Xbp1 expression in ameloblasts may dictate the volume of the ameloblast ER. Supported by NIDCR grant DE016276.