Ion transporters in secretory and cyclically modulating ameloblasts: a new hypothesis for cellular control of preeruptive enamel maturation

Abstract

Mature enamel consists of densely packed and highly organized large hydroxyapatite crystals. The molecular machinery responsible for the formation of fully matured enamel is poorly described but appears to involve oscillative pH changes at the enamel surface. We conducted an immunohistochemical investigation of selected transporters and related proteins in the multilayered rat incisor enamel organ. Connexin 43 (Cx-43) is found in papillary cells and ameloblasts, whereas Na(+)-K(+)-ATPase is heavily expressed during maturation in the papillary cell layer only. Given the distribution of Cx-43 channels and Na(+)-K(+)-ATPase, we suggest that ameloblasts and the papillary cell layer act as a functional syncytium. During enamel maturation ameloblasts undergo repetitive cycles of modulation between ruffle-ended (RA) and smooth-ended (SA) ameloblast morphologies. Carbonic anhydrase II and vacuolar H(+)-ATPase are expressed simultaneously at the beginning of the maturation stage in RA cells. The proton pumps are present in the ruffled border of RA and appear to be internalized during the SA stage. Both papillary cells and ameloblasts express plasma membrane acid/base transporters (AE2, NBC, and NHE1). AE2 and NHE1 change position relative to the enamel surface as localization of the tight junctions changes during ameloblast modulation cycles. We suggest that the concerted action of the papillary cell layer and the modulating ameloblasts regulates the enamel microenvironment, resulting in oscillating pH fluctuations. The pH fluctuations at the enamel surface may be required to keep intercrystalline spaces open in the surface layers of the enamel, enabling degraded enamel matrix proteins to be removed while hydroxyapatite crystals grow as a result of influx of calcium and phosphate ions.

Enamel Pathology Resulting from Loss of Function in the Cystic Fibrosis Transmembrane Conductance Regulator in a Porcine Animal Model

“…These ion transporters include the bicarbonate transporters and chloride channels [Lacruz et al, 2010a]. In enamel organ cells, the particular cellular location of each ion transporter examined is not yet fully appreciated [Josephsen et al, 2010], but based on our own immunolocalization data for AE2 [Paine et al, 2008], and Signal intensity was reduced in the wild-type relative to the null animal to lower the image content out of white (255 grey level) into interpretable morphological details. Wild-type ( e ) and CFTR-⌬ F508 ( g ) enamel comparisons; both images were acquired under identical signal intensity settings.…”

Function and repair of dental enamel – Potential role of epithelial transport processes of ameloblasts

“…The molecular mechanism of this mineral transport process is only partially understood at present. Recently evidence has been presented for specific ion transporters in the ameloblast membrane [7], but the extent to which these are responsible for crystal growth is not yet determined [6]. During the secretory phase the whole thickness of enamel is formed but it is only 30% mineralized.…”

“…This multiple functionality is mainly achieved by PXX amino acid triplet repeats [26]. The uniqueness of amelogenesis derives from the tight junction forming capacity of ameloblasts that isolate apical and basolateral surfaces separating the space of enamel mineralization from the interstitial compartment [7,27], which eliminates the need of large quantities of special phosphoproteins necessary for bone and dentin mineralization [14]. This general role of structural disorder of proteins in biomineralization also highlighted by diseases caused by mutations in biomineralization proteins, such as dentinogenesis imperfecta 1 in the DSPP gene [28] or amelogenesis imperfecta in the AMELX gene [29].…”

“…Unlike other apical epithelial surfaces, for example in salivary glands, pancreas and liver, the enamel surface is practically inaccessible to direct physiological examinations, therefore, information on pH of the enamel in different stages of enamel formation is scarce. But certainly, formation of hydroxyapatite during the maturation stage of amelogenesis generates a large quantity of protons and therefore, to sustain crystal growth, these protons need to be neutralized [7,42,43], most probably by the secretion of neutralizing ions, particularly bicarbonate into the enamel space.…”

Gastrointestinal system

“…Ameloblasts are columnar epithelial cells that line the tooth root in a single layer with basally located nuclei (Figure 3.1). The function of the ameloblasts and the function and nature of their surrounding cell populations changes from the base or secretory region to the maturation region (Josephsen et al 2010). In the secretory region, the ameloblasts are in contact with a layer of flat-to-cuboidal cytokeratin-positive cells, one to four cells thick, the stratum intermedium, which is surrounded by the stellate reticulum and the outer enamel epithelium.…”