UCSF

During amelogenesis (dental enamel formation), ameloblast cells undergo several morphological and functional changes as they form dental enamel. Interference in the timing of these changes results in enamel of poor quality, affecting the psychosocial development of children at a crucial age of their development. However, the mechanisms that facilitate these transitions in ameloblasts are not well understood. Using rodent models as a basis for study, I used microRNA, fluoride, and iron as tools to carry out histological, immunochemical, and quantitative PCR experiments that investigate the mechanisms that regulate and define ameloblast differentiation and function.

As ameloblasts differentiate from one phase to another during amelogenesis, many genes are simultaneously up- and down- regulated. MicroRNAs serve as one mechanism for rapid control of post-transcriptional expression of many genes. Amelogenin, the predominant protein found in the enamel matrix, is alternatively spiced and may serve as a source for microRNAs. I found that exon 4 of the amelogenin gene produces a microRNA that regulates expression of Runx2, a key transcription factor for when ameloblasts are secreting the enamel matrix.

Ameloblasts express and secrete KLK4, a crucial enzyme that facilitates the final phase of amelogenesis. However, it is unclear how ameloblasts begin increasing Klk4 expression at the right time. I found that androgen receptor (AR) serves as an intracellular regulator of expression for Klk4 and that exposure to fluoride inhibits AR translocation to result in decreased Klk4 expression and synthesis.

Normal rodent ameloblasts deposit iron onto the enamel surface, resulting in yellow-brown colored surface enamel. This pigment is lost in rodents sensitive to high levels of fluoride. Iron in the surface enamel is thought to increase wear resistance in the enamel, though it is unclear how ameloblasts acquire, manage, and transport iron for deposition into enamel. I found that fluoride exposure resulted in a significantly increased expression of ferroportin (iron-exporting protein), and significantly decreased expression of the heavy chain subunit of ferritin, which facilitates the safe storage of iron in cells. Furthermore, it appears that changes in these iron-related proteins by fluoride may be changing free intracellular iron levels to affect expression and synthesis of oxidative stress-related genes.

These studies have shown that multiple mechanisms are responsible for ameloblast differentiation. The effects of fluoride on ameloblast differentiation appear to be largely due to cellular effects of fluoride rather than fluoride related changes in the enamel matrix.