Exclusion of all three calbindins from a calcium‐ferry role in rat enamel cells

Abstract

It is widely accepted that healthy enamel formation depends on a steady supply of calcium, yet only fragmentary understanding exists about the mechanisms underlying transepithelial calcium transport. Several lines of evidence indicate that calcium principally follows a transcellular route, which classically is thought to be facilitated by cytosolic calcium-binding proteins termed calbindins. In enamel cells, however, this 'calcium-ferry' dogma appears to fail as we previously found that the major calbindin in murine enamel cells (calbindin-28 kDa) was down-regulated during the peak period of calcium transport and enamel was formed normally in mice lacking calbindin-28 kDa. It remains to be clarified whether the two other known calbindins could function as calcium ferries instead. This study used biochemical and proteomic approaches to obtain definitive identification and quantification of the 30-kDa calbindin (calretinin) and calbindin-9 kDa (S100-G) in enamel epithelium from rat. By establishing that both of these calbindins contribute insufficient calcium capacities in molars and incisors, our results render the calcium-ferry dogma untenable. Of significance to enamel defects and dental bioengineering, these findings support other evidence for an alternative organelle-based mode of calcium transport (calcium transcytosis) and also implicate S100-G/calbindin-9 kDa, but not calretinin, in a calcium-signaling role during enamel maturation.

Calretinin as a Diagnostic Adjunct for Ameloblastoma

“…However, Hubbard et al [22] found that calretinin was primarily expressed during the differentiation stage of enamel formation, and only a faint calretinin band was observed at secretion stage while no such band was detectable during maturation. Based on these findings, the authors suggest retirement of the calcium ferry dogma from enamel biology—in murine models at least.…”

New Paradigms on the Transport Functions of Maturation-stage Ameloblasts

“…Moreover, various calcium-handling proteins were found to be up-regulated ~ 3-fold relative to the secretion phase, closely paralleling the increased calcium flux at maturation (Hubbard, 2000). However, contradicting the "calbindin ferry" dogma long held by the calcium-transport field (based largely on studies of gut and kidney), the major calbindin (Calb1/CB28) was found to be down-regulated during maturation, CB28-null mice lacked the dental phenotype, and the other 2 types of calbindin (Calb2 and S100g) were recently ruled out as alternative calcium transporters (Turnbull et al, 2004;Hubbard et al, 2011). Instead of a cytosolic transport mechanism, the biochemical data invoked an organellar route involving endoplasmicreticulum-related calcium stores (ER/Ca stores).…”

Ca2+ transport and signalling in enamel cells

“…; Hubbard et al . ). For calbindin 9kDa, immunolocalization, radioimmunoassay and mRNA analysis showed higher levels during the maturation stage (Table ) (Taylor et al .…”

“…In enamel cells, calretinin is expressed during the differentiation stage when generalized epithelial cells become pre‐ameloblasts but no data have been reported for secretory or maturation stages (Hubbard et al . ). Mutations to calretinin do not appear to have a major impact in enamel as no dental phenotypes have been reported.…”

“…Hubbard's group had discounted the importance of CaBP as no enamel defects had been identified in mice lacking a number of abundantly expressed CaBP in enamel cells (Hubbard et al . ). This might direct a focus towards an organellar route.…”