The Circadian Clock Modulates Enamel Development

Abstract

Fully mature enamel is about 98% mineral by weight. While mineral crystals appear very early during its formative phase, the newly secreted enamel is a soft gel-like matrix containing several enamel matrix proteins of which the most abundant is amelogenin (Amelx). Histological analysis of mineralized dental enamel reveals markings called cross-striations associated with daily increments of enamel formation, as evidenced by injections of labeling dyes at known time intervals. The daily incremental growth of enamel has led to the hypothesis that the circadian clock might be involved in the regulation of enamel development. To identify daily rhythms of clock genes and Amelx, we subjected murine ameloblast cells to serum synchronization to analyze the expression of the circadian transcription factors Per2 and Bmal1 by real-time PCR. Results indicate that these key genetic regulators of the circadian clock are expressed in synchronized murine ameloblast cell cultures and that their expression profile follows a circadian pattern with acrophase and bathyphase for both gene transcripts in antiphase. Immunohistological analysis confirms the protein expression of Bmal and Cry in enamel cells. Amelx expression in 2-day postnatal mouse molars dissected every 4 hours for a duration of 48 hours oscillated with an approximately 24-hour period, with a significant approximately 2-fold decrease in expression during the dark period compared to the light period. The expression of genes involved in bicarbonate production (Car2) and transport (Slc4a4), as well as in enamel matrix endocytosis (Lamp1), was greater during the dark period, indicating that ameloblasts express these proteins when Amelx expression is at the nadir. The human and mouse Amelx genes each contain a single nonconserved E-box element within 10 kb upstream of their respective transcription start sites. We also found that within 2 kb of the transcription start site of the human NFYA gene, which encodes a positive regulator of amelogenin, there is an E-box element that is conserved in rodents and other mammals. Moreover, we found that Nfya expression in serum-synchronized murine ameloblasts oscillated with a strong 24-hour rhythm. Taken together, our data support the hypothesis that the circadian clock temporally regulates enamel development.

The Divergence, Actions, Roles, and Relatives of Sodium-Coupled Bicarbonate Transporters

“…Investigators studying the circadian rhythm of transcript abundance in mouse molars determined that NBCe1 transcript abundance is nearly doubled during dark periods compared with light periods (537). The authors suggest that this observation correlates with periods of enhanced enamel deposition by ameloblasts (537).…”

Circadian clock-mediated control of stem cell division and differentiation: beyond night and day

“…The observed circadian regulation of adult hippocampal neurogenesis (Bouchard-Cannon et al, 2013) could complement circadian regulation of dendritic spine formation and stabilization (Liston et al, 2013), allowing for more efficient reintegration of new neurons into the adult brain. A circadian clock is also present in tooth ameloblasts, where it controls antiphase rhythms of enamel matrix endocytosis and secretion, as well as ameloblast maturation (Lacruz et al, 2012;Zheng et al, 2013). Circadian oscillations in the release of hematopoietic stem cells (HSCs), coupled with antiphase circadian expression of the chemokine Cxcl12 and coordinated with GSK3β-dependent changes in HSC migratory properties (Lapid et al, 2013), could result in the coordinated release of HSCs and repopulation of the bone marrow stem cell niche (Méndez-Ferrer et al, 2009.…”

The tick tock of odontogenesis

“…Of significance, amelogenin expression still oscillates under DD conditions, however in less extend and with a slight shift in peak times [31]. In vitro studies have also confirmed that amelogenin expression is modulated by clock genes [31, 34, 35] (Fig. 2).…”