Calcium Transport in Specialized Dental Epithelia and Its Modulation by Fluoride

Na+/Ca2+ exchange in enamel cells is dominated by the K+-dependent NCKX exchanger

Calcium (Ca2+) extrusion is an essential function of the enamel-forming ameloblasts, providing Ca2+ for extracellular mineralization. The plasma membrane Ca2+ ATPases (PMCAs) remove cytosolic Ca2+ (cCa2+) and were recently shown to be efficient when ameloblasts experienced low cCa2+ elevation. Sodium-calcium (Na+/Ca2+) exchange has higher capacity to extrude cCa2+, but there is limited evidence on the function of the two main families of Na+/Ca2+ exchangers in enamel formation. The purpose of this study was to analyze the function of the NCX (coded by SLC8) and the K+-dependent NCKX (coded by SLC24) exchangers in rat ameloblasts and to compare their efficacy in the two main stages of enamel formation: the enamel forming secretory stage and the mineralizing or maturation stage. mRNA expression profiling confirmed the expression of Slc8 and Slc24 genes in enamel cells, Slc24a4 being the most highly upregulated transcript during the maturation stage, when Ca2+ transport increases. Na+/Ca2+ exchange was analyzed in the Ca2+ influx mode in Fura-2 AM-loaded ameloblasts. We show that maturation-stage ameloblasts have a higher Na+/Ca2+ exchange capacity than secretory-stage cells. We also show that Na+/Ca2+ exchange in both stages is dominated by NCKX over NCX. The importance of NCKX function in ameloblasts may partly explain why mutations in the SLC24A4 gene, but not in SLC8 genes, result in enamel disease. Our results demonstrate that Na+/Ca2+ exchangers are fully operational in ameloblasts and that their contribution to Ca2+ homeostasis increases in the maturation stage, when Ca2+ transport need is higher.

PMCA Ca2+ clearance in dental enamel cells depends on the magnitude of cytosolic Ca2

Enamel formation (amelogenesis) is a two-step process whereby crystals partially grow during the secretory stage followed by a significant growth expansion during the maturation stage concurrent with an increase in vectorial Ca2+ transport. This requires tight regulation of cytosolic Ca2+ (c Ca2+ ) concentration in the enamel forming ameloblasts by controlling Ca2+ influx (entry) and Ca2+ extrusion (clearance). Gene and protein expression studies suggest that the plasma membrane Ca2+ -ATPases (PMCA1-4) are likely involved in c Ca2+ extrusion in ameloblasts, yet no functional analysis of these pumps has been reported nor whether their activity changes across amelogenesis. PMCAs have high Ca2+ affinity and low Ca2+ clearance which may be a limiting factor in their contribution to enamel formation as maturation stage ameloblasts handle high Ca2+ loads. We analyzed PMCA function in rat secretory and maturation ameloblasts by blocking or potentiating these pumps. Low/moderate elevations in c Ca2+ measured using the Ca2+ probe Fura-2-AM show that secretory ameloblasts clear Ca2+ faster than maturation stage cells through PMCAs. This process was completely inhibited by an external alkaline (pH 9.0) solution or was significantly delayed by the PMCA blockers vanadate and caloxin 1b1. Eliciting higher c Ca2+ transients via the activation of the ORAI1 Ca2+ channel showed that the PMCAs of maturation ameloblasts were more efficient. Inhibiting PMCAs decreased the rate of Ca2+ influx via ORAI1 but potentiation with forskolin had no effect. Our findings suggest that PMCAs are functional Ca2+ pumps during amelogenesis regulating c Ca2+ upon low and/or moderate Ca2+ stimulus in secretory stage, thus participating in amelogenesis.

Unraveled roles of Cav1.2 in proliferation and stemness of ameloblastoma

Background

Transcriptome analysis has been known as a functional tool for cancer research recently. Mounting evidence indicated that calcium signaling plays several key roles in cancer progression. Despite numerous studies examining calcium signaling in cancer, calcium signaling studies in ameloblastoma are limited.

Results

In the present study, comparative transcriptome profiling of two representative odontogenic lesions, ameloblastoma and odontogenic keratocyst, revealed that Cav1.2 (CACNA1C, an L-type voltage-gated calcium channel) is strongly enriched in ameloblastoma. It was confirmed that the Ca²⁺ influx in ameloblastoma cells is mainly mediated by Cav1.2 through L-type voltage-gated calcium channel agonist and blocking reagent treatment. Overexpression and knockdown of Cav1.2 showed that Cav1.2 is directly involved in the regulation of the nuclear translocation of nuclear factor of activated T cell 1 (NFATc1), which causes cell proliferation. Furthermore, a tumoroid study indicated that Cav1.2-dependent Ca²⁺ entry is also associated with the maintenance of stemness of ameloblastoma cells via the enhancement of Wnt/β-catenin signaling activity.

Conclusion

In conclusion, Cav1.2 regulates the NFATc1 nuclear translocation to enhance ameloblastoma cell proliferation. Furthermore, Cav1.2 dependent Ca²⁺ influx contributes to the Wnt/β-catenin activity for the ameloblastoma cell stemness and tumorigenicity. Our fundamental findings could have a major impact in the fields of oral maxillofacial surgery, and genetic manipulation or pharmacological approaches to Cav1.2 can be considered as new therapeutic options.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-022-00873-9.