Dental enamel cells express functional SOCE channels

Abstract

Dental enamel formation requires large quantities of Ca2+ yet the mechanisms mediating Ca2+ dynamics in enamel cells are unclear. Store-operated Ca2+ entry (SOCE) channels are important Ca2+ influx mechanisms in many cells. SOCE involves release of Ca2+ from intracellular pools followed by Ca2+ entry. The best-characterized SOCE channels are the Ca2+ release-activated Ca2+ (CRAC) channels. As patients with mutations in the CRAC channel genes STIM1 and ORAI1 show abnormal enamel mineralization, we hypothesized that CRAC channels might be an important Ca2+ uptake mechanism in enamel cells. Investigating primary murine enamel cells, we found that key components of CRAC channels (ORAI1, ORAI2, ORAI3, STIM1, STIM2) were expressed and most abundant during the maturation stage of enamel development. Furthermore, inositol 1,4,5-trisphosphate receptor (IP3R) but not ryanodine receptor (RyR) expression was high in enamel cells suggesting that IP3Rs are the main ER Ca2+ release mechanism. Passive depletion of ER Ca2+ stores with thapsigargin resulted in a significant raise in [Ca2+]i consistent with SOCE. In cells pre-treated with the CRAC channel blocker Synta-66 Ca2+ entry was significantly inhibited. These data demonstrate that enamel cells have SOCE mediated by CRAC channels and implicate them as a mechanism for Ca2+ uptake in enamel formation.

Enamel: Molecular identity of its transepithelial ion transport system

“…Differences in Ca 2+ handling between the secretory and maturation stage enamel cells are important to better understand their roles in enamel mineralization. Overall, secretory stage enamel cells showed decreased capacity to handle Ca 2+ than maturation stage cells (see also below) [41]. This is also consistent with increased expression of STIM1 and ORAI1 expression in rodent maturation stage ameloblasts [41].…”

“…Ca 2+ could then be removed from the cytoplasm via the plasma membrane Ca 2+ -ATPases (PMCA) [40]. It is now evident that store-operated Ca 2+ entry (SOCE) is a major entry path for Ca 2+ particularly in maturation stage ameloblasts and also to some extent in cells of the secretory stage [41]. SOCE is a common mechanism for the uptake of extracellular Ca 2+ in response to receptor stimulation, generation of the IP3 and release of endoplasmic reticulum (ER) Ca 2+ stores via the IP 3 receptors [42].…”

“…The molecular components of CRAC channels are the ER Ca 2+ sensor stromal interaction molecule 1 (STIM1) and STIM2, and the plasma membrane pore subunit of the CRAC channel known as ORAI1 [44, 45]. CRAC channels are the main mediators of Ca 2+ influx in enamel cells as determined by a reduction in [Ca 2+ ]cyt in thapsigargin stimulated cells pre-treated with CRAC channel inhibitors [41, 46]. Thapsigargin passively depletes ER Ca 2+ stores by blocking the sarco-endoplasmic reticulum Ca 2+ -ATPases (SERCA) therefore stimulating SOCE.…”

Multiple Calcium Export Exchangers and Pumps Are a Prominent Feature of Enamel Organ Cells

“…As future studies continue to better define Ca 2+ export (and also Ca 2+ import—see Nurbaeva et al, 2015a,b, 2017) during amelogenesis, the information in Table 3 will undoubtedly expand and become more precisely defined. Ultimately, elucidating the multitude of mechanisms involved in transcellular Ca 2+ movements in the enamel-forming cells will result in a better understanding of the physiology and formation of enamel, the hardest and most calcified tissue in mammals.…”

Store-operated Ca2+ entry controls ameloblast cell function and enamel development

“…To further confirm deletion of protein expression, tissues from 5-week-old WT controls and Stim1/2 K14cre mice were isolated for immunohistochemical analysis. Our previous immunofluorescence studies reported that secretory-stage ameloblasts do not express STIM1 (15); therefore, we imaged only maturation-stage ameloblasts. Cells from WT mice showed a strong STIM1 signal, whereas no signal was identified in maturation-stage ameloblasts of Stim1/2 K14cre mice ( Figure 1B).…”